2Box uses VST files in their modules for their sounds. So their sounds are more "realistic" than something from Roland or Yamaha. The modules are a PITA to understand, navigate and edit but if you get a kit sounding good that's all that matters, I suppose.
The sounds are recorded samples, not computer generated.
The hit are recorded at multiple velocities multiple times, and the module employs a "round robin" method so you don't get the machine gun effect while playing fast strokes at the same velocity.
It's a great open source system.