When a store item (product) that has a limited supply (i.e. some number available), is placed in the cart by a user, does it, in affect, reserve that item for some period of time?
When an item is placed in the cart, we treat it like the user has reserved that item.
If they did so for an item that only had one remaining, it would mean that no one else could purchase that item.
The “reservation” lasts for 5 minutes. If the item purchase has not been completed within those 5 minutes, the item is returned to the inventory and someone else could purchase the item. The original purchaser, if later attempting to complete the purchase, will be told that the item is no longer available.
This means that when you view the number available in the product area of the Store, the number may not reflect the actual number available, because the item may be in the carts of users but not yet purchased. Only when the item's purchase is complete does the number available get updated.