How does order release determine or material reserve pick which order to process first

We are trying to figure out when multiple orders come in at the same time and one of the orders deplete all of the on-hand qty how does the system choose which order to allocate the item qty to and which one to make a shortage.

Example- The warehouse has 5- widgets on-hand Business Unit A entered an order for 5 and Business Unit B enters an order for 5. They both get approved at the same time and both get auto source, reserve material process, order release request process both orders at the same time (same process instance) how does the system choose which order gets the 5 widgets?