EXFO FTB/IQS-88000 Series Power Blazer for FTB-2/2Pro/500/IQS-600 - Manual (Page 628)

Glossary
616 FTB/IQS-88000 Series
SONET/DSn/SDH/PDH
The FTB/IQS-88000 Series 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-88000 Series 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