Is there a comprehensible algorithm somewhere that determines which fare rules are or aren’t eligible for round trip pricing when you change a partially used ticket? For a simple AAA-BBB-AAA ticket, is it just “as long as you’re going back to AAA, it’s priced as half the round trip fare in whatever bucket you can find?

I’ve also seen plenty of itineraries of type AAA-BBB-CCC-AAA which are ticked as round-trips from a pricing standpoint, same question applies here…

and I’ve also seen itineraries of type AAA-BBB-CCC where it’s priced as a round trip even though the origin and destination aren’t the same…