Rules are organized by priority under Schedules. Priority is "top down", meaning rules at the top of the list have higher priority than those at the bottom.
Priority determines which rule will be checked first. For example, take a Mailbox Parser Schedule with two rules, "Suppress Out of Office" and "Send Email Back to Sender." If "Suppress Out of Office" is below "Send Email Back to Sender" this could create an infinite loop in which automated emails generate automated replies and the mailbox in turn receives new automated responses. To avoid issues like these, it is important to pay attention to rule priority.