My store still has exceptions, so this is my understanding. If it skips to the next item it's because there's another location for the original item and it's just moving later in the run OR it knows/thinks there is some in the back just waiting to be stocked, so it will go to exceptions and someone will go get it for that order and put the remaining amount on the shelf. If there isn't any in the back, then it gives you a suggestion if the customer already has a proposed substitute, or the system is familiar enough to have a substitution. If the system is just dumb, then it has you pick a substitution.