Any item meant to be retrieved directly (not only through a related item) must be an identified item with at least one identifier that is unique within a specified namespace. Since the 2013 edition any scheme that can guarantee uniqueness may be used; Annex A gives the structure when the ISO/IEC 6523-based scheme is chosen.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.