^ A number. We were never really told if she has any real differences from the T-1000. I have theorized that she has a larger weapons range than the other Series 1000 infiltrators (from her attacks at the Kaliba Base) and that she may have a more advanced infiltration software program, but we haven't been told the real differences.
It could be something as simple as she went rogue and didn't want to be identfied as a Skynet unit anymore and added the one to her name.