Originally Posted by
SuperCarnitas
Perhaps the "docs required" online boarding passes print out for destinations where UA has to make sure you have a valid visa (US citizens going to Vietnam, for example) for the itinerary?
That explanation makes a lot of sense. In my experience, I have always got all boarding passes on an online check-in including an international leg (once even with an unexpected op-up!), but in all those cases, I did not need a visa for the destination country.