Microwave oven equipment
Some microwaves have racks so you can cook two dishes at once.  In fact, you can buy generic racks for this very purpose.  However, I also read that some hybrid microwave/convection ovens include metal racks that must be removed if you're using it in microwave mode.  You could check with the manufacturer if you want to know for sure.  They may recommend removing the rack if you're only cooking one dish at a time.