I only ended up in U once, when UA misconnected me and then chose to punish me for missing my connection by cancelling the rest of my itinerary. When I went back online to check my restored itinerary, after a 1K agent had rebooked me, I found that I was now in U (the original discounted booking class being unavailable). I just kinda assumed it was a way of restoring my itinerary without giving me the undeserved privileges for full Y.