What role do you use for learning new fights?

[Bixby]

This applies only to people with multiple 50s, I assume?

That tends to be when the issue first really hits for most people. If there's somebody with multiple 16s stepping into Sastasha for the first time, no reason they couldn't reply (except forum limitations). But yeah, the answer's kind of self-evident for anybody with only one class/job/role.