This object specifies the backward signal indicating that network congestion occurred specifically after the register signaling had already changed over to Group B. The timing detail matters here; this signal only makes sense once Group B signaling is already underway, so seeing it appear before a Group A-to-B transition was ever signaled would indicate a state-machine or protocol-implementation error rather than a genuine congestion event. If congestion hits an exchange right as it begins reporting the called line's status via Group B, this signal is what gets sent back instead of a normal busy/free indication.