At the beginning of an operation, the tool rapids to a point that is a distance away from the beginning of the toolpath. This distance is the Clearance.
The clearance is also used to calculate the move at the end of the operation (except for an ID Groove feature), unless Use clearance as finish withdraw length is deselected on the Leads tab.
The location of these points is also controlled by the Engage Angle and Withdraw Angle.