EXFO FTB-800 Series for FTB-1 - Manual (Page 578)

Glossary
568 FTB-700G/800 Series
SONET/SDH

[0] for AU-4 in STM-1

[A] for AU-3 in STM-1

[A] for the AU-3 in STM-0e, A=0.
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