There is another thread on the board that has developed the Aircraft CAT pretty well. Its going to be based on Vref in a certain configuration. The AC manufacturers certify the CAT, and the State usually accepts this.
It really is per aircraft, as different airlines have different configurations and engines, and of course, ICAO has different FAS than FAA.
Some airlines even use artificially lower max landing weights, to get the regulator to accept a lower CAT for access, or in a case like Ryan, lower landing fees...
Here is Boeings list...the airport CAT includes wingspan CAT and approach CAT.... ie D-III
http://www.boeing.com/commercial/air...oachspeeds.pdf