Just checking for understanding here. Someone correct me if I'm wrong, but this is what I've gathered:
Modules = purchasing planes
Terrains = purchasing maps
Campaigns = purchasing maps with single player missions?
Heh. Good point. The usage is kinda loose.
Common usage is "module" is referring to a vehicle or plane.
But DCS uses a modular approach to add capability onto the base game.
I think you should call anything a module that extends the base game capabilities.
To me planes and vehicles would be "modules", but so would shape asset packs, Command and Control game interface add-on.
Terrains are sort of a gray area. I'd define a terrain as a module.
Missions and Campaigns I'd define as "Content" rather than a module because it is using the capabilities, to provide experience, but is not a capability in itself.
But people willl often refer to a new campaign as a module too.
Modules generally mean plane or vehicle, but could inclue other stuff like Combined Arms interface or object shape packs.
Terrain is a plug-in to extend the base game-play area to other parts of the globe. Dirt, trees, cities.
Missions files are a pre-definition of game-play scenario that utilizes terrains and other capabilities. There may or may not be victory conditions. Stored in a ".Miz" file. A Mission can be designed as SP or MP by what kind of spawn slots are defined into mission definition.
Campaigns are a collection of SP missions that maybe or may not enforce order (success to advance) of play on the mission within or just group them.