A heavy load can mean anything from a fully loaded rucksack to a monster haulbag, in fact anything that’s too heavy to comfortably abseil down with on your back.
The steeper the terrain the greater the need to get the weight of your back, otherwise you will end up hanging upside down. The simplest way to bring the load down is to clip it into the anchor end of your rap sling, so that the load is hanging directly off the descender. With this set up, the bag is kept between the legs.
For a much heavier load, including abseiling with a second climber, attach an abseil device to the load via a sling, then attach your rap sling to the ascender the ropes first through the loads descender, then down to your own, using a prussic back up as usual. Using this method you are hanging from the load and controlling it’s decent via two belay plates, giving great control.