One clarification to add... And this is what I learned here on FT ( ^ ) and confirmed by my own actual experiences.

It is based on Z fare but comes out of R even if R is zero.
So in the above case of AC739, one would get the upgrade based on positive Z space. Once upgraded, it will show R (not Z even though it as R0).

This also applies for international based on positive P but coming out of R regardless of R.

And yes, @mountaintirl, Transborder and domestic are based on Z.