Delivery–first, pickup–second VRPPD. Most researchers make the assumption that
customers can be divided into linehauls (customers receiving goods) and backhauls (customers
sending goods); furthermore vehicles can only pick up goods after they have finished delivering all
their load. One reason for this is that it may be difficult to re-arrange delivery and pickup goods on
the vehicles. We note that such an assumption makes the implementation issue easier, since
accepting pickups before finishing all deliveries results in a fluctuating load. This may cause the
vehicle to be overloaded during its trip (even if the total delivery and the total pickup loads are not
above the vehicle capacity), resulting in an infeasible vehicle tour.