P & D Charges help automate pay distribution when a driver is part of a "split load".
A split load is a probill that is picked up on one trip and delivered on another. Consider a total revenue of $100: If a particular pickup to a drop point is only 10 miles, then Fleet Manager may only allocate $5.51 to that portion of the leg, then the additional hook to deliver leg will receive the remaining $94.49.
We account for this when you pay a driver based on probill revenue. If the driver of the deliver leg gets 80% of the revenue then this will be based on the revenue for the second pickup to deliver, which was established above as $94.49 - so the driver will receive 80% of $100 (total revenue) subtracting $5.51 (allocated revenue from first leg) for a total of $75.59 being paid to the driver of the second leg (80% of the 94.49 allocated revenue of second leg).