For an example, here is something from HL2 news, " Next week we will be releasing the Source SDK" I also saw there is a DOOM 3 mod called SDK I think, so what is SDK.
No! I'm Spamacus!
17th June 2003
Software Development Kit It allowes modders to really delve into the secrets of the game and fool around with the engine that runs the game (I am pretty sure). Games like Battlefield 1942 did not have an SDK made available so you have a lot of newer mods running extremely slowly because they had to use an engine that didn't really support the complexity of the mods.