You may have found that certain weapons/curios in Darktide have modifiers stating things like "10% Damage (Infested Enemies)".
Or perhaps you're confused by the difference between whatever the hell a "Dreg" or "Scab" is.
The following is a list of the game's potentially confusing enemy titles and variations:

-Scabs
-Dregs
-Elites
-Maniacs
-Infested
-Unyielding
-Specialists
-Groaners
-Poxwalkers
-Unarmored
-Flak Armored
-Carapace Armored
-Monstrosities

This guide acts as a simple breakdown of which enemies in specific fall into each category, as some terms cross over one another. This guide operates under the assumption you already know what each specialist is and what it does, so I won't be going into detail as to how enemies like Flamers or Pox Hounds work; just what they're classified as for stat modifiers.

Before we get into armor types, I highly recommend you check out this imgur page showing how the game determines damage modifiers. Most of you will get more utility out of viewing this than this guide:
https://imgur.com/a/Selj5nI/layout/grid

Essentially, there are 6 classifications of "armor" applicable to enemies: Flak, unarmored, carapace, maniacs, unyielding, and infested. Some enemies only fit one; others have two or three applicable varieties. Hitting an enemy in a specific hitspot in the Psykhanium will show you what type the afflicted area counted as.

Carapace Armor - Only the Crusher is entirely labeled as "carapace-armored". Outside of that, the Mauler, Bulwark, and Reaper wear Carapace armor in varying areas. The Mauler only has a carapace helmet, the bulwark has carapace armor on its arms and lower legs, and the reaper has carapace armored lower arms, lower legs, and a shoulder pauldron.



Flak Armor - Flak armor is worn by most soldier varieties. Enemies entirely adorned with flak armor include: Bombers, Renegade Captains, and Scab Ragers. Beyond this, all other scabs wear flak armor on their torso and lower legs, and most have helmets. Melee scabs lack helmets and are unarmored on the thighs, scab gunners are fully armored except for their thighs, scab shotgunners have exposed arms and thighs, scab stalkers (no gasmask) only have armor on the torso and head, and scab shooters (gasmask-wearing gunmen) only have exposed shoulders and thighs. As for dregs, both the gunner and shotgunner have armor on the torso but nothing else. Dreg gunmen are fully unarmored, and melee-wielding dregs only have a flak armor helmet.



Unarmored - This one is fairly straightforward, however it's important to note that "visibly exposed flesh" =/= "unarmored". Enemies entirely labeled as "unarmored" include: Snipers, Groaners (See Poxwalker category), and Dreg gunmen (melee dregs have a flak armor helmet).
Outside of these, see the flak armor category above for armored soldier's exposed areas.



Unyielding - This category is a bit arbitrary. Unyielding is essentially applied to larger enemy's exposed areas, however it does not count as "unarmored". Unyielding entirely applies to: Beasts of Nurgle, Plague Ogryns, and Daemonhosts. Reapers and Bulwarks are "Unyielding" in their exposed areas, but carry flak and carapace armor elsewhere.
It is important to note: Crushers, Mutants, Maulers, and Ragers are not unyielding.




Maniacs - This category applies to a theme of specialist enemies that is essentially, bonkers nuts. Their AI is designed to charge into the fray regardless of their personal safety. All maniacs are entirely covered by the "Maniac" hitzone type. Despite visual armor on flamers/trappers, they accept the "maniac" damage modifier no matter where they're hit. Maniacs specifically include: Dreg Ragers, Flamers, Tox Flamers, Mutants, & Trappers.



Infested - This category refers to enemies more profoundly afflicted by Nurgle's finest diseases. Like maniacs, all infested enemies are entirely covered by the 'infested' hitzone, with no exception. Infested enemies specifically include: Pox bursters, Infested Poxwalkers (see Poxwalker category), and Pox Hounds.




TL;DR: Maniacs and Infested refer to specific enemies independent from armor classification. Carapace, Flak, Unarmored, and Unyielding damage modifiers do not affect Maniacs and Infested enemies.
Unarmored, Flak armored, Carapace armored, and Unyielding are different tiers of "Armor" that can apply to different areas of an enemy's body. Damage modifiers apply to what type of armor you are shooting; Shooting a mauler in the head with a carapace armor bonus will apply the damage bonus, but not in their chest, as their chest armor is flak.

Specialists - Specialists are enemies that spawn into the match with special attacks- hence the name specialists. They are enemies with a special gimmick to them: area denial (flamers, bombers), single-target takedowns (pox hounds, trappers, mutants), or glass cannons (snipers, bursters).
Specialists include: Bombers, Flamers, Mutants, Pox Bursters, Pox Hounds, Snipers, Trappers, and Tox Flamers.

Elites - This term refers to enemies that are essentially more heavily armored or carrying stronger weapons. They can also be found wandering maps unaware of the player until provoked, unlike specialists who actively hunt you as soon as they spawn.
Elites include: Bulwarks, Crushers, Gunners, Maulers, Ragers, Reapers, and Shotgunners.

Scabs refers to the traitor guardsmen of the Moebian Sixth.
Dregs refers to corrupted civilians and basic cultists without formal military training/equipment.
The terms "Scab" and "Dreg" can apply to enemies of both specialist and non-specialist varieties. For now, I will explain how to identify the non-specialist varieties.

Basic scab soldiers are identifiable by gray, often spiky armor, green, gray, or brown faded clothing, armored melee dudes without helmets, and lasguns.
Basic dreg soldiers are identifiable by yellow/tan clothing, a lack of armor, hand/footwraps, bare-chested melee dudes with shiny metal helmets, and autoguns (traditional firearms).

Scabs and dregs both have melee and ranged variants. The game randomly switches between spawning these two varieties, though the easiest way to tell the difference is this:
If you're being attacked by lasers or melee dudes wearing armor but no helmets, they're scabs.
If you're being attacked by gunfire and melee dudes wearing helmets but no other armor, they're dregs.

"Poxwalkers" is generally treated as the all-encompassing term for Darktide's zombies, however there are two subvarieties: Actual poxwalkers and groaners. The difference is that the "poxwalker" is entirely of the Infested armor variety (See Armor category), while groaners are of the Unarmored variety. Additionally, poxwalkers have more HP than their groaner companions.
To avoid confusion, I will be properly referring to the infested zombies as poxwalkers and the unarmored zombies as groaners. Outside if this guide however I don't think anyone cares if you, me, or anyone else calls both "poxwalkers" for brevity's sake.

If you wish to tell the difference, poxwalkers have green skin, large horns/mutations, very little clothing, and stumble about. Think "late-stage rotting zombie." Groaners are more recent victims; they have more clothes, blue/purple/pinkish skin, complain about maggots under their skin and how hungry they are, they yell insane ramblings at you, and they don't often have horns on their head- if they do have horns, they're notably smaller.


An example of weapon stat bonuses that can apply to these enemies include:
+15% Damage (Poxwalkers, Groaners)
+15% Damage (Infested Enemies)
+15% Damage (Unarmored Enemies)

The following information may be incorrect. Take it with a grain of salt.
After testing weapon damage in the psykhanium, I believe that the "(Poxwalkers, Groaners)" damage modifier seems ONLY to apply to groaners, NOT poxwalkers. I tested a low-damage rapier and high-damage lasgun with ONLY this damage modifier; both routinely did higher damage to the groaners compared to poxwalkers (rapier did 35 - 38 damage to poxwalkers, 47 - 51 to groaners. Lasgun always instant-killed groaners and did over 100 damage, but sometimes did less than 100 damage for poxwalkers). This is weird as this damage modifier seems to be meant specifically for both zombie types. However, this may also just be that the Infested armor type has more general resistance than Unarmored, and that both were still receiving additional damage from the modifier as they should.

The (Infested Enemies) modifier ONLY applies to poxwalkers, pox hounds, and pox bursters.
The (Unarmored Enemies) modifier applies to groaners and a variety of other enemies but does NOT apply to the Infested poxwalkers. (See Armor category)

Monstrosities - The game's penances and general chatter routinely refer to bosses as "monstrosities". Monstrosities include: plague ogryns, beasts of nurgle, daemonhosts, and traitor captains. However, most of these fall under the "unyielding" armor category in regards to weapon damage modifiers, with exception of the traitor captain being flak-armored.

Bombers & Bursters - As far as I'm aware, neither of these titles conflict, though this may be confusing for some players. "Bombers" in regards to curios damage reduction modifiers seems to refer solely to the bomber specialists, not pox bursters, and vice-versa.

Gunners & Reapers - Reapers count as Gunners and thereby, Gunner resistance curios work when being shot by either.

"Adhesive Charge" for the Rumbler does not stick to Traitor Captains, despite them being monstrosities.

Just to be extra sure you get how this works, here's a few examples to better clarify:

Lets say you just bought a revolver from the requisitions store. The revolver does 300 damage per shot, and has a "+20% Damage (Carapace Armored Enemies)" modifier. This means:
It WILL do +20% damage when shooting a Crusher anywhere, and it will also do +20% damage when shooting a Mauler in the head, or a Reaper/Bulwark's arms or leg areas.
It WILL NOT do extra damage to any other enemy, nor will it work against flak armor, such as shooting the Mauler anywhere except the head, shooting a Reaper's flak-armored chest or exposed areas, which count as "Unyielding".

If the same revolver had a "+20 Damage (Unyielding Enemies)" modifier instead:
It WILL do +20% damage to Beasts of Nurgle, Plague Ogryns, Daemon Hosts, and Reapers/Bulwarks if you're shooting their exposed flesh.
It WILL NOT do +20% damage to ANYTHING else, nor will it work if you shoot the Reaper / Bulwark in their armored areas, such as the Reaper's chest.

If the revolver had a "+20% Damage (Maniacs)":
It WILL do +20% damage versus Mutants, both Flamer varieties, Trappers, and Dreg Ragers.
It WILL NOT do +20% damage versus anything else, nor Scab Ragers.

By now I think you get the idea but please comment if you have any questions!