Half of you are toddlers who don't like to read, so here's the picture version of this guide:

Here's a word summary of above if your attention span is slightly better.

Models
You can only have one, you cannot combine two models together by subscribing to two model addons, no matter how little they change.

Textures
Two types, VTF and VMT. VTF files usually change colors or patterns on the thing it replaces like most survivor reskins. Survivors have separate textures for the body and head, allowing for mixing and matching those. VMT files work with VTF, these don't replace colors, only give textures different functionality like RNG or reduced reflections.

Weapons
All or nothing, weapons come bundled with animations so you cannot combine a reanimation mod with a remodel. Custom textures work fine for reanimations.

Non-weapon Animations
Survivors and infected have their animations stored in a separate file, which is why xdR works with remodels. Mods that come with custom animations built in do not support reanims for those specific animations (or any if all animations are made to be built in).

Now that we've sifted through roughly 95% of the game's playerbase, let's get some details.

First off, what does it even mean when an addon is conflicting?
It does not mean that upon loading into a map the game will instantly crash and Gabe Newell will personally come to your house and burn it down, instead it simply means that the two addons are replacing things with the same filename.

For instance, the filename for Nick's suit textures is "gambler_body.vtf". If you have one mod that makes the suit itself black and another that makes the shirt red, this does not mean that they will combine into one. Since they would both have to modify "gambler_body.vtf", you would only see the effects of one in game.

But which one you ask?

That depends on the load order, a.k.a which addons have priority over other ones. Whatever addons you subscribe to first will take priority over any you subscribe to after. To modify this load order, follow this guide:
Right click on Left 4 Dead 2 in your library and click "properties"



Then go to the workshop tab in the window that opens



Ensure you have "Load Order" selected in the sorted by. Moving the addons with the three lines will allow you to modify which addons take priority over others

The bane of anyone who does model edits (especially Zoey ones)'s existence is the question "Why doesn't this remodel work with another remodel?"

Well, like said above, both remodels modify the same filename. Models in Left 4 Dead 2 are not broken into parts* so you can't combine two remodels just by subscribing even if they change completely separate things.
Hey look its them again.

Although the default hair is untouched in Fashionable Zoey, making you think that combining it with something that only touches the hair would work, these two conflict as they both change Zoey's model file "survivor_teenangst.mdl".
Simple as that, if you really want to combine them then crack out some elbow grease and make the combination yourself, hell you might find yourself enjoying doing it.

*If you missed it, this asterisk was on "parts" in the second sentence. Playermodels and most props in the game are made up of only one part so combining doesn't work, some models however are broken into multiple parts. As in, several different models with different filenames are put together to make one large thing. This is mostly done for big destruction stuff like the Dead Air plane crash or larger areas like the No Mercy map 4 & 5 skybox.

This is why something like this:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3174844565
and this:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=450256600
don't conflict as although they're both changing the No Mercy skybox, they change the different models that make it up and so there is no conflict of filenames. You'll have to use some common sense when judging whether something like this applies to other similar mods.

For reasons still not understood by scientists today, in map 1 of The Passing, the Zoey and Francis models that talk to you from the bridge use a completely different model and filename.

This is why something like this:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2197947211
can sometimes work with other remodels (although most come with the light models for whatever they're changing already included, it usually is mentioned in the description of them).

In the Source Engine, textures are made up of two parts, the VTFs which contain the actual pictures of what you're gonna see it, and the VMTs which tell the engine how to render these pictures.

It can be hard to tell what changes just a VTF and what changes both. Usually survivor textures only edit VTFs.


All the default survivors are made up of two* main VTFs, the one for the head and the other for the body. If an addon only changes one, then you can mix and match it with another that changes the other. However, just like with models, you cannot combine two body textures or two head textures even if they change minor separate things from each other.


This ♥♥♥♥♥♥♥ image was like a kilobyte too large to upload (microsoft paint came in clutch)

This here is the VTF for Nick's body texture. What you'll notice is that it contains the pants, shoes, suit, shirt, rings and even his hands. Since these all make up the "gambler_body.vtf" file, you cannot combine two body textures together even if they change different things since they'll both contain all of this.

*Another asterisk, on "two". Survivors are made up of the head and body VTFs but also have extra ones.
-All survivors use separate VTFs for their eyes but these are shared across certain survivors (like Louis and Francis share the same eye VTF)
-Nick, Coach, and Ellis also use "producer_body.vtf" which is Rochelle's body texture. They use this for the holster.
-Nick, Rochelle, Ellis, and Zoey use an extra VTF for the transparent sticky out bits on their hair.
-Bill and Francis use an extra VTF for the transparent hairs on their beards.

You don't see many mods that ONLY have the VMT files, as edited VMTs are usually used to supplement edited VTFs.


Despite both of these appearing to change how the deagle looks like, they don't conflict. This is because the less shiny one modifies the VMT and the gold texture modifies the VTF.

However, like everything else, you cannot mix and match VMTs just by subscribing. Here is a general list of things you'll likely see on the workshop that edit VMTs so you know what you can and can't use together:
-Adding/removing reflections
-Adding/removing phong
-Making textures animated
-RNG textures
-Adding fancy RGB colour effects

Weapons are a weird case where the animations for them are built into the model, rather than being separate (this gets touched on in the animations section later).

Since the animations are built into the model itself, you cannot combine a remodel with a reanimation as they both modify the same files.

Weapons are also broken up into two parts, the "viewmodel" and "worldmodel".
The viewmodel is the thing you see in first person, the worldmodel is the thing you see in thirdperson or on other survivors or on the ground.
This is why you can use something like this:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3261534962
alongside any mod that changes the model of the M60 while on the ground (although good luck finding a worldmodel-only weapon mod).

Like any other models, the textures are separate VTFs and VMTs so you can mix and match those. Official Valve weapons usually only have 1 VTF for the entire gun, so you cannot mix and match certain parts of them.

Particles are made up of two things, the textures that the particle effect itself uses and the PCF file the particle is housed in.

There are a variety of PCF files in the game already that are grouped rather arbitrarily that each house a variety of particle effects in them. Mods that alter just one particle effect from these will have to edit the entire PCF.
Here is a list of all of them, don't ask me what's in them cause I don't know. Particle stuff isn't my thing.
steamworks
environmental_fx
rain_fx
burning_fx
fire_01
fire_01L4D
fire_01L4D_garage
fire_01_lighthouse
electrical_fx
steam_fx
locator_fx
witch_fx
speechbubbles
rain_storm_fx
military_artillery_impacts
rope_fx
spitter_fx
fireworks_fx
vehicle_fx
insect_fx
charger_fx
item_fx
gen_dest_fx
tanker_explosion
firework_crate_fx
coldstream
infected_fx
environment_fx_river
environmental_fx_river
fire_01_river
fire_01L4D_river
weapon_fx_river
laststand
________

Some addons choose to add new PCF files instead of overwriting the existing ones. The issue with that is that particles are loaded through what is called the "particles_manifest" text file. Not only is the name awesome, it contains the filenames of each of the PCF files the game loads when you play. The issue is that you can only have one of these so one mod that adds new particle effects will likely conflict with another that adds different new particle effects.

I'll admit, I'm not too well versed in these myself but I get the gist.

Unlike most other models, the survivors and infected have their animations contained in a separate "model" that is then just implanted into whatever their actual model is. This is why something like xdR (an addon which allows for reanimations) works on custom models, as they edit two completely different things. Weapons and props come with their animations built into the model and so you cannot combine different animations with a different model.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2763369547
This addon by Cuba does not work with reanimations. Despite the witch storing her animations in a separate model file, this addon has these animations built into the witch model to be used for stuff that is only required of the addon itself.

As a rule of thumb, mods that use animations that either don't have xdR in the name, or make something happen with the model as the animation plays, likely won't work with reanimations.

Some smaller stuff that didn't warrant an entire section.

Soundscapes: These are usually stored alongside many others in a text file alongside similar ones (like for the same campaign or general area type like rural or urban) so two soundscape mods that edit the same campaign's likely will conflict but those that affect different campaigns won't.

Sounds: Each sound has it's own file, duh. These are sometimes shared amongst several things like the slide being pulled back is shared between the deagle and default pistol.

Color Correction: Each is a separate file, so can be mixed and matched with other color correction mods that don't edit the same one.

HUD: A lot of factors, a lot of stuff that I have no clue on. The HUD is made up of many different text files so some HUD mods can be combined (but not all).

Talker scripts: The txt file ones conflict with each other as they change the same files. The VScript ones (like those by me) don't conflict as they use scripts rather than those text files.

Vscripts: Some nuance to this that I don't feel like going over. For the most part all Vscripts will work with each other.

Physics: The ragdoll of a model is stored in a separate file that ends with .PHY. As such, you can merge remodels and rephysics(?) together. This is assuming the remodel doesn't include a custom PHY file, but a lot of workshop creators add one even if they don't change anything :(