It happened to us today as follows:

My wife and daughter are flying ORD-PHL-LHR today. They have United Award Tickets and the ORD-PHL leg is UA while PHL-LHR is US. I checked them in at the UA International Kiosk, checked their luggage and got their boarding
passes. As we were walking to security, I noticed that the second
boarding passes said "paper required" on them. Now the last time we
ever had a paper ticket, it was because multiple airlines were involved. But we certainly received no paper ticket for this award, and why would the need for a paper ticket start on the 2nd leg of the itinerary?

So I called mileage plus. I got two agents who couldn't help me, and didn't know what it meant. Then we found a first class check-in agent who wasn't busy and asked her. She said that it meant that they would have to fill out a form at the gate in Philadelphia.

I'll find out later what form they fill out or what happens there and add to the thread.