If your question is restricted to United, then the answer is as follows:

First (Free) = XF/"R"
First (UG) = NF/"X"
Business (Free) = XC/"O"
Business (UG) = NC/"I"

Note that there are actually two booking classes for each award category.

That's because United's internal systems allow some special two character booking classes (XF/NF/XC/NC) which don't work in most other booking engines including large GDSs, e.g. Abacus, Amadeus, Apollo, Sabre and Worldspan and/or major web sites, e.g. Expedia, Orbitz and Travelocity.

In either case, it's almost impossible to look at inventory levels for these booking classes.