Plane change (also known as "change of gauge") means that you have to get off the inbound plane and switch to another plane for your connection, which might even be at a gate all the way across the terminal. The crew and your seat are completely different -- the only thing in common is your flight number. If your inbound flight is late, your outbound flight won't necessarily be held. As I can recollect, you can't even choose seats for anything beyond your first segment on united.com.

Does anybody know why UA does this so often? It seems like such a farce with no apparent benefit, and it decreases the miles/EQS you earn. I also understand that you cannot upgrade individual segments without upgrading the whole thing. We have "direct" flights to Japan and Brazil out of BOS using this system, even though the A319 headed to ORD obviously isn't going transpacific.