The two-letter booking classes for award classes have been there ever since they went to dynamic awards and is how they control booking at the various award levels. This is in addition to fare rule requirements which include things like advance purchase requirements, roundtrip booking with min stay, etc. (which can be found in Award fare rules). But they generally have only shown the two letter award class when going to the flight details listing for the award as shown below (while top-level flight option listings have typically only shown base award class).
Note that both Basic and Main awards start with 'N' class, but actual two letter award booking classes are NE for Basic, and NY/NB/NM/NH/NQ/NK/NL/NU/NT/NX/NV for Main.




There is also dual inventory checking going on for awards. In example below, there is only NL bucket for flight in Main (and books into YSL220 22K Main award level on JFK-DFW route -- note that different routes will have different mileage amounts for a NL class award). Note the the 'L' in the third letter of the award fare basis code which correlates with the NL booking class availability.





For a C+ award on flight, even though there is only a single SW booking class, the fact there is only NL booking class available in Main forces the C+ award to be a 31.5K WSL315 award. The third letter in the fare basis code is used to determine award level for dual inventory checks.




Both the YSL220 Main awards and WSL315 C+ awards have a 14 day advance purchase requirement as found in the award fare rules for them. So these award levels won't be bookable within 14 days of flight even if there is NL booking class open on flight.

All these concepts (fare class, fare rule requirements, and dual inventory checks) basically translate directly from revenue fares and is how they are able to closely correlate awards mileage levels with revenue fare prices.