Originally Posted by sbm12

The letters define inventory buckets that are used to sell the seats at different prices and with different rules. The "R" bucket is for award upgrades as you've noted. To check the inventory the easiest thing is to do a search for the flight you're looking for and click on "fare class" on the right column of that display matrix. That will show you all the fare buckets and the number of seats in each. If you are using Chrome or FireFox with GreaseMonkey installed there is a script here that shows you all the fare buckets without needing to click and it highlights the award ones.