EXFO FTB/IQS-85100G Packet Blazer for FTB-500/IQS-600 - Manual (Page 408)

Glossary
398 FTB/IQS-85100G
SONET/SDH
The FTB/IQS-85100G supports numbering SONET low order path using the
two-level “VTGroup#,VT#” convention for numbering VTs within an STS-1.
For example: VT1.5 [1,3], VT2 [3,2], VT6 [6,1].
The FTB/IQS-85100G supports numbering SONET high order path STS-nc
within an OC-N using the two-level “STS-3#,STS-1#”. For example: STS-12c
[5,1].
SDH Numbering Convention
As per ITU G.707, the high order paths are defined using a 2 to 5 level
convention E,D,C,B,A depending on the rate of the STM-n used.

E: the AUG-64 are numbered 1 to 4

D: the AUG-16 are numbered 1 to 4

C: the AUG-4 are numbered 1 to 4

B: the AUG-1 are numbered 1 to 4

A: the AU-3 are numbered 1 to 3
So for the naming is as follows for each of the following rate:

[E,D,C,B,A] for STM-256

[D,C,B,A] for STM-64

[C,B,A] for STM-16