A master resource identifier. The IANA PEN [PEN] provider ID SHALL be specified in bits 0-31, the least-significant bits, and objects created by that provider SHALL be assigned unique IDs with the remaining 96 bits. <br/>0xFFFFFFFFFFFFFFFFFFFFFFFF[XXXXXXXX], where [XXXXXXXX] is the PEN, is reserved for a object that is being created (e.g., a ReadingSet for the current time that is still accumulating). <br/>Except for this special reserved identifier, each modification of an object (resource) representation SHALL have a different "version". <br/>