Yup, I try to explain it with shouting a message to someone in a crowded room. You can yell at your boss "I fixed the bug", they can confirm it or ignore you, w

Yup, I try to explain it with shouting a message to someone in a crowded room. You can yell at your boss "I fixed the bug", they can confirm it or ignore you, which is delivery at most once if you don't repeat the message. If you try to repeat the message until they confirm it, it is at least once delivery. edit: Point is in confirming that message is received. If you don't receive the confirmation the message was delivered at most once.