Behavior of Disallow Back Order in Checkout for Matrix Items

Setup> Site Builder> Setup Website> Shopping subtab> Web Store Out Of Stock Items = Disallow back orders but display out-of-stock message

1. When out of stock matrix item is purchased, there would be an error message in the Review & Submit Order page:

Unable to Checkout. 

We are unable to place this order because one or more items in your order is not available in sufficient quantity. Please go back and reduce your order."

2. And the Submit Order button is disabled.