Originally Posted by
JALPak
No, it's not. They have a few patterns to choose from. The SHA based CA on KIX-LAX flight mentioned by OP won't be flying NRT-LAX e.g.
Believe it's YVR, ORD and CDG for HKG crew. These routes/patterns change over time. They used to put HKG crew on NRT-LAX too and CDG was added a couple years ago.
I wouldn't use the word "choose", since they cannot make a choice. But there is a pattern scheduled by JAL.