We are currently trying to place an A/C article that I wrote last June to this site. It will do a better job explaining belt slip.
Basically the compressor control system looks for belt slip by monitoring the speed of the compressor (speed sensor in rear head) versus the speed of the motor. The point to all this was to anticipate a compressor that was locking up. If such a thing were to occur the belt would be destroyed and all the other accessories would quit turning; especially the water pump (not such a good thing).
The computor software verion is only important for reprogramming.
The variant code is capable of being changed at the head and can be very important. It is the way that one can change from temp in Degrees C to degrees F. The improper coding of a gas motor as a diesel will keep the compressor from coming on. This is one of the cautions mentioned somewhere in this thread.
Bosch Master, ASE Master, L1
33 years MB technician