EXFO FTB-800 Series for FTB-1 - Manual (Page 579)

Glossary
NetBlazer 569
SONET/SDH

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