Mods: Server Only vs Server Client vs Headless vs Optional
After you deployed the mods you have to select which mods are to be loaded for the server profile.
Here you have to decide if you want load the mods as SERVER ONLY, SERVER + CLIENT HC (Headless Client) or OPT (Optional), but what does this actually mean?
Note
In 95% of cases, selecting the desired mods within the SERVER + CLIENT collumn is what you want.
If you plan to use a headless client, you can simply copy the mods from client to headless.
This is the most common, the default case.
When a mod is selected in the SERVER + CLIENT column, it means it will be loaded on both, the server and the clients.
For example, if you want to load a mission that uses certain units, vehicles etc, you need to have those assets available on the server and on the client. Same goes for mods like ACE, TFAR and most other mods.
If Key Verification is enabled, these mods will be required to load via the launcher.
(as long they have a biskey file and you didn't forget to copy the keys).
When a mod is selected in the SERVER ONLY column, it will be loaded only on the server.
Examples usually are mods that are focused heavily on scripts that only run on the server, but will never be executed on the client.
Caution
ONLY load a mod as Server Only if you a >9000% sure, that the mod in question is made for this and you know what you are doing!
Using this option on unsuitable mods will provide you with horrible issues that are terrible to debug.
Trust me.
Examples for these are: Simple Craters, Advanced Towing, Advanced Sling Loading, or other mods that might affect AI behavior.
The simple rule of thumb here: Unless you really know what you're doing, for example due to the mod description or other indicators, you will not need to load a mod as a "Server Only" Mod.
Headless Clients are the arma-way of multi-threading for servers.
A headless client can take-over AI from the Server Process itself and handle all the needed stuff regarding. These AI Calculations - for example: Line of Sight, Path finding etc. can be very expensive to run, espl. if your scenario has a lot of AI.
To do that, it needs most mods like any regular player-client, but there are a few exemptions, that can be omitted. Those mods usually are considered cosmetical, soundmods or player-utilities. Examples are the following: DUI, Shacktac, JSRS, ...
Important
Mods like Blastcore which effectively change the particles of smokegrenade should be considered to be run on headless clients as well as the server.
If these mods change the particle effects - especially those who have the property "canBlockAiVision". Not loading these mods on the processes that control the AI, you may end up with AI that is percieved as "being able to see trhough the smoke" because they will simply see the vanilla smoke, which might be much much smaller.
Useful for any mod that can be an optional, but not required client-side mmod.
This will add the keys of those optional mods to the servers keys directory without making loading them on the server/making them mandatory for the connecting player-clients.
Examples for these are - like with HC - mostly cosmetical but not gamechanging mods, like DUI, Shacktac, JSRS, ...
FASTER is only dealing with the key verification that is in Arma. ACE whitelist is a different systems and requires you to put your Optional Mods in the PBO whitelist still.