Quake II deviated from Quake I convention and packaged game code within a dll (gamex86.dll), rather than a custom format. This meant that building a mod for Quake II was as easy as building and supplying a custom dll for the game.
PlatPak was the very first Quake II mod ever released, shortly after Quake II's release in 1997. This mod modified the weapons to be far more deadly and more closely resemble the weaponry of the original Quake.
PlatPak II builds upon PlatPak. It replaces the bullets of the machine gun with rockets.
Note that each of these builds requires a very specific patch of Quake II to run.