Seawolf,
I'm a reservationist with UA. Permitted open jaws are determined by the unflown segment. We look at the fare of that unflown segment and it must be equal to or less than the two segments you want to fly. If so the other segments are allowed.
An example of an open-jaw that would not be permitted is if a person wanted to fly from BOS to PHL then DEN back to BOS. The "unflown" segment PHL to DEN is a higher fare than the other segments - therefore cannot be done. Hope this was clear.
The unflown segment (PWM