A substrate is whatever an object is made of, in the broadest sense: matter, field, signal, or some condition for which the institution has no settled word. Many accessions are compatible with ordinary storage. A significant minority are not — they decay, contaminate their neighbours, translate themselves into other forms, or quietly disagree with the physics of the room they are placed in.

Incompatible items are isolated by mount, atmosphere, field boundary, or — where no physical measure suffices — by jurisdictional fiction, a custody arrangement that holds an object apart by agreement rather than by wall. Compatibility tests are small, slow, witnessed, and stopped at once when a sample begins to explain itself.

Transfers between substrates require a declared destination state and a return condition accepted by the Registrar before the first test is run. No accession is moved toward a more convenient substrate merely because the convenient one is easier to store.