I work on exactly this problem, it is actually pretty complicated. A grocery store does know how much is delivered but it does not know how much is in stock. A

I work on exactly this problem, it is actually pretty complicated. A grocery store does know how much is delivered but it does not know how much is in stock. A simplified version is like this. 20 boxes of cheerios come in, there are room for 5 on the shelf, there are room for 5 on top of the shelf, then 10 go to the backroom. Someone steals a box from the shelf, another takes a box and puts it back behind the frosted flakes. Now the inventory is showing 20 boxes, but there are only 19 in the store, and one of those 19 is somewhere hiding somewhere. These are the easy cases. Consider shirts. A shirt can come in different sizes or colors but they all have the same upc number. We have 20 shirts according to inventory, no need to order more right? The only problem is that all the mediums and larges sold out and we only have xxls and xsms. Also items can come in packs, but are sold individually. There can be special displays, so some of the items are moved from their old location to a new location. On and on with edge cases and exceptions. The parent comment on most stores have no idea is closer to the reality than stores know because they know what came in on the trucks.