Identifies the TP being switched away from.

-  In case of 2F MSSPRING, this is the TP that switched. The East and the WEST side can switch independently. This is one of the TPs that switched.
-  In case of 4-Fiber rings, when a span switch occurs, this is one of the TPs in the MSP ( 1+1, 1:n) groups (worker or protection).
-  In case of a ring switch, this is the worker TP of the span that switched. For example if the EAST span switches to the WEST span then this TP is the East - Worker.
-  In case of SNCP, this is the TP that the traffic source switched away from.

Readable by NMS? Yes
Writeable by NMS? No
Invariant? Yes
