The SCS must give each revision an identifier unique within the repository. Where the identifier is a digest of the revision's content (as in git), nothing further is needed; where it is a number or other non-digest, the SCS must document how the revision's immutability is ensured. The same revision identifier may appear in several repositories.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.