Access Control Matrix
The theoretical model underneath every authorization model — subjects as rows, objects as columns, and each cell holding the permissions a subject has on that object.
No real system stores the matrix as a literal grid — it’s sparse and enormous — but every model is a strategy for compressing it. Slicing by column gives access control lists attached to each object; slicing by row gives capabilities carried by each subject; RBAC, ABAC, and the rest are further compressions that avoid listing every cell by hand.