Yes, when explained as the "superset" thing, it becomes hard to understand. So it's easier to think in term of keys and locks:
Incoming DP is your entry door into a building. At entry, you're are given all the keys that are in the incoming list, that in fact can be no keys at all (empty list).
To exit the building, you take an exit door (outgoing DP), and you need to have at least a key that opens one of the locks (listed in outgoing COR). If you are not allowed, you can try (rehunt) another DP with a compatible destination-pattern, a lesser preference and one key/lock match. This is why COR can also be used as a mechanism to route based on calling number (answer-address).
Finally, if either entry or exit door has no list, you're allowed anyway. This concept is crucial for reducing a COR configuration to the minimum necessary.
Thanks for the nice rating and good luck!
