EXFO FTB-8510B Guia Del Usuario página 1269

The low order paths are defined using a 2 or 3 level convention K,L,M
depending on the rate of the AU-4 or AU-3 used to multiplex the low order
signals.
K: the TUG-3 are numbered 1 to 3
L: the TUG-2 are numbered within the TUG-3 0 or from 1 to 7
M: the TU-2, TU-12, TU-11 are numbered within the TUG-2 1, 1 to 3, 1 to
4 respectively
SONET/SDH Application
Glossary
SDH Numbering Convention
619