Cathay Pacific Asia Miles - Fare codes for AAdvantage award tickets

Can anyone clarify what fare classes AAdvantage award tickets book into?

I'm looking at an AAdvantage business-class BOS-HKG award ticket, and the fare code is U on the CX legs. (The AA codes are U for 3-class segments, Z for 2-class segments.)

F on a 3-class flight is also Z.