SOP / SOP Instance UID

Service-Object Pair — pairs an information object with a service; each instance has a unique SOP Instance UID.

A Service-Object Pair (SOP) binds an IOD (the object, e.g. a CT Image) to a service (e.g. storage). The SOP Class UID says what kind of object it is; the SOP Instance UID is the UID of this specific object. Derived copies must get a new SOP Instance UID so they don’t collide with the original.