Cogging is where the stepper motor actually misses a step. At that point, the controller thinks it has moved the motor X steps, but the motor cogged Y times, so the motor has actually moved X-Y steps.
The D48E, D100E, and D300E pan tilt units (PTUs) all have encoders on the motors. If the user enables Encoder Correction (CEC), then the PTU will detect slips. When a slip is detected, the PTU will come to a stop and then immediately attempt to complete the move. This action will be repeated until a new command is issued to the PTU.
At all times the reported position will be correct. A user can query the number of times that the unit has stopped and restarted with the CPEC and CTEC commands (encoder Correction Pan Error Count and Correction Tilt Error Count).