Back in '16 when I ran them, RJL's weren't specified as SS, ZZ or XX but they were generally known as high initial bite and that's why I liked them. Popular with the SV crowd or anyone else running weak "classic style" brakes.
But now I see this image describing the versions of the RJL's. Interesting.
I have to wonder if it would be worth trading off a little of the initial bite of the ZZ for the high friction coefficient of the XX.