SKUU calls this customer-hosted endpoint to set the absolute quantity for one variant at the assigned virtual SKUU_Network location. An identical write is a successful no-op. This endpoint never changes the customer's physical stock. Return exactly HTTP 200 and exactly the three request fields with unchanged JSON types and values. SKUU retries transport errors, 429 and 5xx responses up to three times using the identical signed body; other 4xx responses are permanent. Honour a numeric Retry-After header when supplied.