This Standard provides requirements for the design, fabrication, inspection, testing, marking, and stamping of pressure vessels for human occupancy, having an internal or external pressure differential exceeding 2 psi. This Standard also provides requirements for the design, fabrication, inspection, testing, cleaning, and certification of piping systems for PVHOs. A PVHO is a pressure vessel that encloses a human being within its pressure boundary while it is under internal or external pressure that exceeds a 2 psi differential pressure. PVHOs include, but are not limited to, submersibles, diving bells, personnel transfer capsules, decompression chambers, recompression chambers, hyperbaric chambers, high altitude chambers, and medical hyperbaric oxygenation facilities. This does not include nuclear reactor containments, pressurized airplane and aerospace vehicle cabins, and caissons.