A warhead bus is a structure that holds the warheads of a MIRV ICBM after the missile has finished boosting, and has left atmosphere. It is a framework that contains computers, gyros and other mechanisms all designed to release individual RVs at precise times and angles in order for them to hit their targets at various points on the ground. Typically, after the last warhead has been dispensed, the bus will burn up during re-entry. It can also do more mischevious things like deploy decoys or chaff, burn flares to create false thermal signatures, etc.