Every institution is a tenant. Every record it owns is bound to that institution's account.
Isolation is enforced at two independent layers.
A record cannot be created without an owning account, so an unattached record cannot exist.
And every read of tenant-owned data is restricted to the tenant making the request, even when a query does not request it. A defect in one layer is caught by the other.
The tenant is determined from the authenticated request itself. It is never taken from anything the caller supplies.
Before any endpoint runs, the request is checked against that tenant. The tenant must be active. The domain and the credentials must belong to it. And the client address must be permitted.
The address check is fail-closed. An address that cannot be read is refused, not allowed.
Departmental separation rides on top. A sub-account scope is honored only for callers who are entitled to it.
What this is, precisely: isolation by account, enforced at the application and data layers. Isolation reviews are recorded per module, against each milestone.