There was a question about why certain letters like N and Q were used but seems that Question has been deleted. For those still looking for the answer its also in Annex 11
2. Composition of designator
2.1 The ATS route designator shall consist ofa basic designator supplemented, if necessary, by:
a) one prefix as prescribed in 2.3; and
b) one additional letter as prescribed in 2.4.
2.1.1 The number of characters required to compose the designator shall not exceed six characters.
2.1.2 The number of characters required to compose the designator should, whenever possible, be kept to a maximum
of five characters.
2.2 The basic designator shall consist of one letter of the alphabet followed by a number from 1 to 999.
2.2.1 Selection of the letter shall be made from those listed hereunder:
a) A, B, G, R for routes which form part of the regional networks of ATS routes and are not area navigation routes;
b) L, M, N, P for area navigation routes which form part of the regional networks of ATS routes;
c) H, J, V, W for routes which do not form part of the regional networks of ATS routes and are not area navigation
routes;
d) Q, T, Y, Z for area navigation routes which do notform part of the regional networks of ATS routes.
2.3 Where applicable, one supplementary letter shall be added as a prefix to the basic designator in accordance with
the following:
a) K to indicate a low-level route established for use primarily by helicopters;
b) U to indicate that the route or portion thereof is established in the upper airspace;
c) S to indicate a route established exclusively for use by supersonic aircraft during acceleration, deceleration and
while in supersonic flight.
2.4 When prescribed by the appropriate ATS authority or on the basis of regional air navigation agreements, a
supplementary letter may be added after the basic designator of the ATS route in question in order to indicate the type of
service provided in accordance with the following:
a) the letter F to indicate that on the route or portion thereof advisory service only is provided;
b) the letter G to indicate that on the route or portion thereof flight information service only is provided.