They stand for the stepping level of the processor. When AMD (or Intel) make changes to the processor, they give the processor a new "stepping level" so that you know which version it is. The CG stepping level is newer (and improved) over C0.
Supposedly, the boxed, retail version ADA3000AXBOX 3000+ is a CG, the ADA3000BOX might not be. Not sure if that is really tue or not.
You can tell the stepping level by looking at the top of the chip. There are software utilities that let you check, too.