Relationship Tuple
The single fact Zanzibar-style systems store — object, relation, subject — like "report:Q3
aka: Zanzibar tuple
A relationship tuple is deliberately the only kind of fact the model stores — no roles table, no permission matrix, just a graph of these triples. Userset rewrites let one relation compute from another (a report’s “viewer” can be defined as a team’s “member”), which is how a single new tuple can ripple into access for an entire group at once.