EXFO FTB-8100 Series Transport Blazer for FTB-500 - Manual (Page 549)

System Tab
SONET/SDH Application 535
Default Test Preferences
For ODU2 background traffic (ODU0 client in ODU1):
The above example shows that ODU2 that contains ODU0 in ODU1
foreground traffic uses ODU0 background traffic. The remaining
tributaries use ODU1 background traffic.
For ODU3 background traffic (ODU1 client):
The above example shows that ODU3 that contains ODU1
foreground traffic uses ODU1 background traffic.