Originally Posted by
ORDnHKG
It is not about UA, or flights to US. There is someone stand right at the security line (there they check boarding pass and passport, also looking at the size of your bag), if they see your carryon kinda look big, will ask you to put your carryon bag to see if it fit inside the cage. If it doesn't work, probably go back to UA counter to ask for an exemption tag.
It's an HKIA airport requirement - don't know why they do this but it goes back to the old Kai Tak airport days. In any case suggest you ask the check in agent when you check in whether or not there will be any problems. Offhand I don't think there would be but if there is, they will have colored tags that you attach to your carryons that will allow you to bring your bags on board.
BTW be aware that on UA flights leaving HKG, there is an additional security check at the jetway itself after you submit your boarding pass at the gate. Security will go through your carryons for anything problematic but that's about it.