[Pierre-Marie Baty]

There is additional code in the single-player source that is used to build a game and a client DLLs that support single player missions (which multiplayer source does not). Single player DLLs differ a bit from multiplayer DLLs because they need to implement rather complicated things that are specific to single-player gaming, such as save/restore features and seamless transition between maps.

In short, only use the single-player source if you want to code something that will make use of single-player features ; use the multiplayer source for all the rest.