Subsystems are similar to your typical system, there is a difference though. Consider these systems to be smaller systems inside of systems. They can have introjections, headspaces, alter roles, anything a normal system has it can have. The type of subsystem depends on the system it's within and who's in it. You sometimes share a main headspace body with subsystems. This is similar to how outside of headspace everyone shares a body. There are two types of subsystems that I'm aware of, DID subsystems (Which have dissociative barriers like in normal DID systems) and OSDD subsystems, which lack these barriers and allow for sharing memory naturally.