Custom modfolders is a concept in Operation Flashpoint that allows addon makers and players to completely change most aspects of the game. They were introduced with the release of the resistance addon, which also became the first total conversion modification.
It has become a common practice to start mod folder names with a "@" in front of the name to distinguish them from other files in the Operation Flashpoint directory.
Making it Work
To load and use a modfolder it has to be added to the games startup parameter "-mod". This is easiest done by creating a new executable shortcut and editing the startup parameters.
If for example you are trying to load a mod named @test the startup command might look like this:
"C:\Program Files\Codemasters\OperationFlashpoint\FLASHPOINTRESISTANCE.EXE" -mod=@test
To load several mods separate their names with the ";" character.
How it Works
The "-mod" parameter defines a path the engine will use to load data files when loading the game. If running OFP, it is important to remember that the RES mod will automatically be added first to the list of mod folders (OFP only). A mod folder can contain all the data that your OFP / ArmA install folder can contain. This includes missions, campaigns, and addons subfolders, etc. The most common use of modfolders is to organize addon .pbo files.
Since most players today have a very large number of extra addons installed it has become a common practice to divide them into addon modfolders. The benefit of this practice is that the engine only has to load the addons in the modfolders specified by the player, lowering memory usage significantly. To create an addon modfolder simple pick a name and create the directories "@myname\Addons\" in the Operation Flashpoint directory. Next place all addons you want in this modfolder into this directory and add "@myname" to your "-mod" list when these addons are needed.
To make this easier you might wish to create several shortcuts loading different modfolders.
The main drawback of this technique is that it's hard for other players in multiplayer games to know what kind of changes you are playing with as modfolders can be used to make beneficial configuration changes to the game, in other words cheat.
Conflicting addons / addon load order
In ArmA, it is possible for addons to modify classes in other addons, including those in the game's core "addons". If the user installs two addons that try to modify the same property of a given class (for example, the model property of the basic soldier class), then obviously both addons can not work as intended (together in the same install, that is).
In these cases, there is almost nothing that the user can do to determine which addon takes "priority". Inside of the addons themselves is a property called "RequiredAddons", which determines addon load order, regardless of modfolder order.
A more in-depth discussion of this can be found here.
Alternatively, you can use 'OFP Mod Selector', by Operator of Dissidence.