Optional external reservation reference, typically the booking id in your own system. Supplying it makes the create idempotent: within one Hostex account a given channel_id can only ever produce one reservation, so a create that is retried after a timeout cannot result in a duplicate booking. Submitting the same channel_id again returns the existing reservation together with its current status instead of creating another one — check status rather than the HTTP status code, since a replay of a reservation that has since been cancelled also returns 200. The value is stored as the reservation's channel_id and can be queried back through Query Reservations. Uniqueness is scoped to the Hostex account, not to your application or to the custom channel, and a value stays claimed permanently — including after the reservation is cancelled. Letters and digits only — no punctuation of any kind, including hyphens, underscores and dots.
