There may be a hard and fast rule but AC certainly goes to great efforts to make sure we don't know what it is. Being open and honest with customers is not a corporate philosophy for this organization.

The general consensus is that status trumps fare except (maybe) if you have a connection - in which case the IT monkeys at AC run a special algorithm involving the position of the moon and the number of bowel movements amongst AC senior management.

And to think those US airlines actually post boards at gates so that pax can see where they stand in the upgrade order. What sort of fun is that?