IIRC many websites (e.g. for buying concert tickets) have a lock mechanism where you have X amount of time to make your purchase during which time only a limited number of people can be in the checkout process.
Sequential assignment follows strict order of payment completion only. No race conditions, no held inventory, no time windows. You either complete the transaction and receive the next sequential number, or you don't.
The constraint is designed to eliminate the entire apparatus of purchase optimization, including queue management systems.