Creates one fulfillment order for the named open order lines (each quantity at most what no open fulfillment order covers; 409 exceeds_open_quantity otherwise) and reserves its open quantity at its queue in the same transaction. An externalId an open, untouched fulfillment order of the order without an external id already covers line for line is ADOPTED (that fulfillment order takes the external id: no second fulfillment order for the same lines). An externalId already on the channel follows onDuplicate (default update; error 409 duplicate; skip answers the stored one).

Scope: fulfillment:write, channel:write or admin. A permission group made before the Channel API scopes existed grants it as fulfillment-orders:manage.

Idempotency: send Idempotency-Key; a retry with the same key replays the first answer.

Rate limit: write class, 1,200 requests per minute per tenant by default (read class 6,000). Every response carries X-RateLimit-Limit, X-RateLimit-Remaining and X-RateLimit-Reset; a refusal is 429 rate_limited with Retry-After.