MT literature is full of confusion and it is about time to decide and choose proper (adequate) names

M0B - macromolecular pool content relative to liquid pool M0A=1

f=M0B/(M0B+1)

interestingly, f is more physical but at the same time does not scale linearly with molecular content

R and k represent MT exchange rates. If you define R you only need one constant, with k you need two kAB and reverse kBA

kAB=M0B*R

finally A and B or "free" and "solid" etc?

Maybe we should vote?

For historical reasons I would vote for M0B and R (the ones chosen by Henkelman  in the first paper on qMT)