Flow control is an admission layer for shared GPU inference. Before a request gets routed to a model server, it's either admitted or held. Under saturation, priority decides which class of traffic advances, fairness spreads dispatch turns across tenants so no one gets starved, and ordering picks the next request. Lower-priority work waits instead of crowding out what matters. Admission decides when a request runs. Routing decides where. Keeping those separate is the whole idea.