Compartment edge

Definition (canon)

A surface across which a system keeps separate copies of its own state while still passing something through a limited channel, placed where exchange across it is slow compared with relaxation on either side. It is read from the system's own dynamics, never from a name or a population norm.

Three self-referenced tests: (1) separate copies, in space or in genotype; (2) limited channel, each side's state independent of the other's given what crosses; (3) timescale ratio, exchange time ÷ internal relaxation time ≫ 1.

Three kinds: fixed, metered (the channel has an aperture), transferable (an endpoint's membership changes at a commitment event). Always qualified.

In plain words

A boundary that keeps two separate versions of a system's state, with a narrow channel between them, where crossing is slow compared with settling on either side.

It is not

• A membrane. A membrane is one way to realise a compartment edge, the metered kind.
• A pool, which is individuated logically, not spatially.
• A paper's scope. A scope is a boundary statement and counts only where its edges pass the three tests.

What would defeat it

• Follows from the definition. A claimed edge that fails any of the three tests when measured against the system's own dynamics.