I've been told that it's mostly automated. IE the Point Of Sale tells the system what items were sold and it subtracts from whatever total it has. The produce guys have said that because of this system some things like bananas are almost always screwed up, cause the barcode and the PLU (the numbers) aren't actually the same item. Namely sometimes when the banana suppler changes they don't get automatically ordered cause the system thinks we're selling the old ones which "don't exist anymore". I'd expect similar things happen to grocery and home but to a much less extent, probably from people just tossing stuff out instead of scanning it out and then still just tossing it. Yay food waste.