From united hub:
A note about passports: There are two changes to the check-in process that you should be aware of if you’re used to traveling with United:

1. If you’re a United customer and are booked on an international flight, you no longer need to enter your passport information manually when you check in. To store your passport information for later use during online check-in, you must first swipe your passport at an airport kiosk or hand it to an airport representative for validation. This means that customers that didn’t have their passport stored in the Continental system will not receive their boarding pass right away until their passport has been swiped or verified by an agent.

2. You can store only one passport at a time in our system.