A split happens at one of three moments, and the moment decides everything else. It can happen at the swipe, when a shared card debits several people’s accounts at once. It can happen at the request, when one person pays and everyone else gets a notification. Or it can happen at the receipt, when someone reads the itemized check and works out who actually had what.
Those are not three interfaces over the same idea. They are three different trades. The earlier you divide, the more certain you are of collecting — and the less you know about what you are dividing. At the swipe the money moves itself, but the split ratio was agreed before the check existed, and the amount that authorizes is not guaranteed to be the amount that settles. At the receipt you can see every line, the tax and the tip — but the money already left, so all you have left is an ask.
Information and enforcement mostly run in opposite directions, and the products that imply otherwise are quietly choosing for you. There is one real exception, and it is not an app: a restaurant’s own point-of-sale can divide by item and take each payment in the same moment. Its price is that it happens on the restaurant’s terms, at the table — and on Toast’s system in the US, past nine ways the staff drop to a manual workflow.