One reseats and lives to relieve again. The other opens once, stays open, and has to be physically replaced. That single difference decides which one belongs where.
A spring-loaded relief valve (PSV/PRV) holds a disc against a seat with a calibrated spring; when upstream pressure exceeds the set pressure, the disc lifts, vents overpressure, and — once pressure falls back below the reset pressure — the spring reseats it, ready to relieve again on the next event. A rupture disk is a thin metal (or graphite) diaphragm engineered to fail — to burst — at a specific pressure. It has no moving parts and nothing to reseat: once it bursts, the opening stays fully open, venting the entire vessel inventory, and the disk itself must be physically replaced before the system can be safely returned to service. Both devices exist to protect equipment from overpressure. They are not interchangeable, and the choice between them is one of the more consequential decisions in process safety design.