ICAO Annex 11
A,B,G,R for routes which form part of the regional network of ATS routes and are not RNAV routes;
L.M.N,P for RNAV routes which form part of the regional network of ATS routes;
H.J,V,W for routes which do not form part of the regional network of ATS routes and are not RNAV routes;
Q,T,Y,Z for RNAV routes which do not form part of the regional network of ATS routes;
Prefixes - K for low-level helicopter routes; U to indicate theroute is established in upper airspace; S to indicate a route established exclusively for supersonic aircraft
Suffixes - Y for RNP1 routes at or above FL200 to indicate all turns between 30 and 90 degrees shall be made within the allowable RNP toleraqnce of a teangential arc between the straight leg segments defined within a radius of 22.5NM; Z for RNP1 routes at or below FL190 to indicate all turns between 30 and 90 degrees shall be made within the allowable RNP toleraqnce of a teangential arc between the straight leg segments defined within a radius of 15NM; F to indicate that on the route advisory service only is provided; G to indicate that on the route information service only is provided;
There is method in the apparent madness