Gear in Closers has a noticeable effect in PvP, however not all means of improving your character stats will transfer over to PvP. While in PvP, character stats will be hard capped at the following values:

-Physical Critical Damage: 200%
-Magical Critical Damage: 200%
-Skill Cooldown Reduction: 40%
-Physical Damage Reflect: 15%
-Magical Damage Reflect: 15%
-Attack Speed: 15%
-Movement Speed: 20%*
-Conditional Critical Damage Bonus: 15%
-Conditional Damage Increase: 15% softcap, hardcap unknown
-Empowered state: 30%

There is no limitation in +skill tunes and bonuses from items.

It is possible that some other stats are capped, but their cap is too high to reasonably attain with current gear, and thus have not yet been found. These are the currently known ones.

Nothing is capable of breaking these boundaries, be it through character passive buffs (such as Exploit Weakness Bits), class buffs (such as Phase Ductility), active skill buffs (such as EML) or Teamwork and Crew Buffs, these limits cannot be exceeded by any means.
It must be noted that some stats see severe normalisation from PvE values to PvP. While the cap for Air Strike Damage Increase is 15%, Conditional Damage bonuses are decreased by 75%~ while in PvP. This means that to hit the 15% hard cap for Air Strike Damage Increase in PvP, one would need to have at least 60% pre-normalisation (in PvE).

The higher a stat, the more normalisation it incurs. In the case of Conditional Damage, having 7% in PvE translates to 3.38% in PvP (52%~ normalisation), whilst having 60% in PvE translates to the hard cap of 15% in PvP (75% normalisation). The precise formula is unknown; however it is certain that the following stats see severe normalisation:

-Conditional Damage Increase: 60% in PvE required to soft cap, sees more severe normalisation after 60%
-Empowered State Damage Bonus: 90% in PvE required to hard cap
-Physical and Magical Defence: 70-75% normalisation from PvE to PvP (in average)
-Empowered State Duration: doubled from PvE to PvP

While in PvP, character levels are adjusted to maximum, which serves to normalise any significant level difference. Skills however, are not. Therefore higher level individuals benefit from stronger skills and passives due to having more skill points to allocate. Higher level individuals also benefit from better gear, which allows them to reach higher damage values and come closer to the stat caps. Although stronger normalisation is incurred by higher stats, highly refined weapons and modules will see a palpable advantage over weaker gear. Tunes also apply, which make them a significant factor in PvP gearing.

As raw Physical and Psi Power has no hard cap, a person using a +11 Weapon Core will do less damage than a person using a +13 Weapon Core because their raw Physical or Psi power will be lower overall. On the other hand, excessive Critical and Conditional Damage (which yields higher Total Combat Power) will not apply.

* - Although Movement Speed is capped at 20%, this applies solely to percentile increases. Gazelle Type is not affected by any caps and stacks with % increases, which means that Gazelle Type tunes do work in PvP and can scale quite well. Characters like Levia, Tein, Seha, etc (any who have buffs that grant them 20% mspeed) can attain the mspeed cap without the use of items, however other characters may need to spec for it with items like No Eyes Doll or Camilla Doll.

General PvE/Dungeon-style restrictions still apply in PvP. This means that one cannot change gear or their skills while in a PvP match, with the added restriction that hotkeys which would otherwise open menus (such as P for Profile in default) are disabled, and menus must be manually opened through the Esc UI. Chatting is disabled in PvP, and you cannot write to your enemy, reply to whispers or type in Crew Chat while in the arena (unless in 3s, where chatting is enabled for Party and Crew, if in Crew 3s). Aside from this, there are certain buff and item restrictions in PvP.

The following things do have an effect on PvP:

-Item Refinement Bonuses
-Item Passive Bonuses (most yellow-coloured text present in Core/Modules/Trinkets/Receivers, such as +15% Crit Rate for instance)
-Item Passive Set Effects (such as PG75s +15% Conditional Critical Damage effect on 3-piece)
-Gear and Costume Tunes
-Gear and Costume Set Effects (such as Dark Command’s 10-piece effect, granted that it does not exceed the stat cap)
-Regular Skill Cubes (Basic, Advanced and Master, bar few exceptions depending on character)
-Option Skill Cubes
-Chip System
-Crew and Teamwork Buffs
-Active Skill Usage Buffs (such as Sputnik, Zumwalt or Follow Me! granted that they do not exceed the Stat Cap)
-Titles

The following things do not have an effect on PvP:

-Consumables (cannot be used)
-Item Active Effects and Buffs (for example, 10% chance to increase Air Strike Crit Damage from the 3/3 Charismatic Trinket Set, or Tiamat’s Raw Module’s ‘on mission start’ Damage Increase, often also yellow coloured)
-Chip Active Effects and Buffs (aka every Platinum ‘on chance’ Chip)
-Item ‘on cast’ Effects and Buffs (such as Disaster Modules’ ‘upon using x skill’ procs)
-Categorical Damage (Players are no longer classified as ’Unarmoured’ and instead have no classification, rendering passives such as Transcendental Power and categorical damage Receiver tunes useless)
-PNA Stats
-Consumables
-Talismans
-Pets

Skills with a ‘restand’ property are capable of picking up an enemy from the ground. Here follows each character’s restand abilities:

Seha:
-EX Shockwave
-Dash (Aerial down key variant)

Sylvi:
-Gravity Flux (On 1st and 3rd use; 3rd use has 20%~ smaller hitbox than 1st use)
-EX Gravity Flux (It'll restand on cast but knockdown once the damage ticks; must be cancelled out of before the slam to restand)
-Telekinetic Explosion (Grab, only on explosion/blowback)
-Special Move: Bus Drop

Yuri:
-Spin-Kick Rapid Shot (Only on Ground usage, no directional input)
-Phase Flip
-EX Spread Shot (Only on Aerial usage)

J:
-Kung-Fu Hustle (Only on Aerial usage)/EX Kung-Fu Hustle (Works on both Aerial and Ground usage)
-Groundbreaker
-Neck Grab (on follow up kicks after success, both ground and aerial)
-Special Move: Morning Aerobics (2nd use, on ground pound)

Misteltein:
-Niflheim (2nd use, on crystal eruption)
-Fenrir

Levia:
-Hell’s Trap (Grab/SAC Skill)
-Serpent Waltz
-Tail Whip (on success/pull up, useful in desync and laggy scenarios)
-Special Move: Black Sabbath (2nd use if ground, 3rd use if aerial)

Nata:
-Sure Kill (with stacks)
-Guillotine (On rising slash)
-Night Hunting

Tina:
-Cool Your Head
-Pot Shot
-Special Move: Peacemaker (Only on Aerial usage, also on 2nd use with the machine gun throw explosion)
-Special Move: Finish Them (Only on final hit, ridiculously situational and practically impossible to use in most scenarios, but it's a restand alright.

Harpy:
-Ventus Impact
-Sonic Boom (kick followup on successful aerial use, third kick on ground use)
-Special Move: Lunatic Typhoon (on 2nd usage/wind wave followup)

Violet:
-Help Me, Hyde!
-Cutting Rose (on downward swing)
-Dashing Jump Attack (on exceed/semi-exceed state, can be refreshed)
-EX Camilia Drop (on additional input if no Master cube)

Grabs are skills which will bypass Super Armour Level 1. Although grabs can catch through Combat Dodge (as it only has SA1), some grabs can be Perfect Spaced while others cannot. Perfect Spacing grabs is difficult to do so consistently due to desync and inadvisable, however it is possible.

Here follows each character’s grabs:

Seha:
-Point Blank (Will dash from current aerial position to ground, enabling range extension depending on height)

Sylvi:
-Telekinetic Explosion (Range is drastically affected by aerial position)

Yuri:
-Spin-Kick Rapid Shot (All except up key variant when in ground use)

J:
-Neck Grab
-Biomagnetism
-Clothesline (Will dash from current aerial position to ground, enabling range extension)

Misteltein:
-Tornado (Ground variant has wide y axis, aerial variant has wide z axis)
-Mjolnir

Levia:
-Hell’s Trap (2nd use, immediately after restand; can be positioned from range)
-Tail Whip

Nata
-Lasso (1st and 3rd hits of the skill counts as its own grab, meaning even if the first strike is immuned, the final pull attack will regrab)

Tina:
-Close Combat
-Sharp Shooting (on dive/dash)

Harpy:
-Sonic Boom
-Cyclone

Violet:
-Burst Blade
-Stinger

It should be noted that Special Moves all have natural SAC2, which makes them capable of staggering through SA (functioning as pseudo-grabs themselves).

Grab Immunes allow the user to become completely immune to grabs during their duration. These do not persist and cancelling the animation with another skill will immediately make you susceptible to being grabbed to lingering grabs (namely Biomag, Tornado, and Hell’s Trap). If the grab immune does not have a long enough duration, even if you successfully immune, grabs with lingering or multiple hitboxes may sometimes still stagger if they outlast the immune duration (such as Nata's Lasso or Tornado).

Here follows each character’s grab immunes:

Seha:
-Phase Aura Blade
-Fire Grinder
-Empowerment

Sylvi:
-Gravity Flux/EX Gravity Flux
-Space Elasticity

Yuri:
-Lunging Slash.

J:
-Kung-Fu Hustle/EX Kung-Fu Hustle
-Health Drink
-EX Hundred Fist
-EX Biomagnetism

Misteltein:
-Release
-Fenrir

Levia:
-Summon Familiar
-Flash of Pain
-Hell's Trap

Nata:
-Shadow Hunting

Tina:
-Pot Shot

Harpy:
-Draw
-Cyclone (only on Ground usage/pre-dash)

Violet:
-Exceed (active usage/skill buff)
-Reinforced Blow

It should be noted that Special Moves all have natural SAC2, which makes them capable of staggering not only through regular SA, but also through every single grab immune.

Special Moves also have SA2, which means that their casting animations double up as grab immunes/one cannot be grabbed during a Special Move.

Invulnerabilities/iframes are a state in which a character is completely immune to anything and everything. This includes grabs, which make iframes double up as grab immunes as well as 100% damage resists. Here follows each character’s iframes:

Seha:
-Point Blank (Only on successful hit; can be extended or finished early)

Sylvi:
-EX Supercluster

Yuri:
-Spin-Kick Rapid Shot (On successful grab)
-Spin-Kick Rapid Shot (Ground use up key variant)
-EX Spread Shot
-Lightning Fast (Sheathing sword animation)
-Special Move: Five-Point Strike (First half/pre-followup)
-Special Move: Power Lunge (Full duration if not charged)

J:
-Neck Grab (On successful grab)
-Kung-Fu Hustle (Only on aerial usage, and only before rising from the ground on kick)
-Special Move: Swan Song (During tornado portion)

Misteltein:
-Special Move: Valhalla’s Court

Levia:
-Snake Signet
-Designated Sacrifice (Full invulnerability from frame 1 until the ending animation of the first blow)
-Tail Whip (On successful grab)

Nata:
-Endless Pursuit (Lasts through all 5 attacks, can be delayed to extend)
-Shadow Hunting (Lingering fall iframe on hit)
-Night Hunting (During upwards zigzag)
-Special Move: Purgatory

Tina:
-I Want To Be Alone
-Trick Shot (0.6 seconds per shot cast)
-Dive (Only during movement, expires on landing)

Harpy:
-Special Move: Icarus Fall

Violet:
-Post-Initiative (has lingering fall iframe on aerial usage; early iframe loss with no Super Armour on ground usage)
-Stinger (has iframe until the followup swing throw and Hyde assist - procs on any hit, be it successful or immuned, but not on whiffs)
-Special Move: Efflorescent Dance

Damage Reduction are skills in which the character will take less damage than normal. Here follows each character’s DR abilities. Unless stated otherwise, all DR skills have 80% reduction:

Seha:
-Phase Aura Blade
-Fire Grinder
-Empowerment
-Pulse Cannon/EX Pulse Cannon (Can be extended through delayed followups)

It should be noted that all of Seha’s True Damage abilities will completely bypass any DR.

Sylvi:
-Gravity Flux/EX Gravity Flux
-Wormhole (During teleport animation)
-Electric Storm
-Psychic Satellite

Yuri:
-Superspeed
-EX Phase Flip

J:
-Hundred Fist/EX Hundred Fist
-Kung-Fu Hustle/EX Kung-Fu Hustle
-EX Biomagnetism

Misteltein:
-Aegis Rush/EX Aegis Rush
-Tempest
-EX Mjolnir

Levia:
-Snake Signet (Even when the iframe falls off, the animation retains 80% DR)
-Hell’s Trap
-Fang Bolts
-Serpent Waltz

Nata:
-Skin Slayer
-Torrent of Pain

Tina:
-Pot Shot
-Phantom Snipe
-Rapid Fire (35%~ DR)

Harpy:
-Harpy Strikes
-Zephyr Combination

Violet:
-Violet has passive 60%dr on *all basic attacks and command normals* performed while in Exceed/Semi-Exceed state
-Exceed: Charge
-Stinger
-Cutting Rose

It should be noted that all FMs also have passive 80% DR during their cast animations.