When functioning normally, the tip moves in some direction off its axis, dependent on the angle it contacts the surface that is being mapped, and forces a circuit to open. The probing plugin recognizes this and responds by retracting the probe in the positive Z direction. Normally this would remove the displacment force the part is impacting on the tip and the tip would return to its centered (in the probe) position and close the circuit. However, if the tip does not move enough to reclose the circuit (i.e., it's stuck open), the plugin will continue commanding the probe to move up (+Z) in an effort get the circuit to close. This would be bad if you don't have limit switches in this direction. To avoid this the plugin asks for a maximum Z value if stuck to stop the carriage return if this condition should arise. If this condition should occur, the probe tip would rise no higher than the value you enter. Ideally, you would set this height some distance just above the highest point on your part.
Hope this helps.