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.
To save time and more importantly frustration, have them give you the exemption tag right when you check in if you are worried about it. I have always asked and they gave it to me with no problems