How does Oracle determine what subinventory to allocate to when manually reserving using the schedul

We have multiple allocable subinventories within our org.  Our use case is that an order management user needs to allocate available material to a sales order so it won't get allocated by pick release for other orders we don't want it to release for.  So, the user goes into the scheduling organizer, queries by item number.  Scrolls down and types in quantities in the "qty reserved" field and clicks "reserve". 

My question is when the user clicks "reserve", how does oracle determine what subinventory to allocate that from?  Is it by the picking order value tied to each subinventory?  Alphabetical order? Both? Something else?