This is used in the outgoing VC ID field within the Virtual Circuit FEC Element when LDP signaling is used, or the PW ID AVP for L2TP. This is the identifier the two PE routers must agree on for LDP or L2TP signaling to correctly correlate one PE's pseudowire configuration with the matching pseudowire on the other end, so a VC ID that does not match on both sides is one of the most common reasons a pseudowire fails to come up despite both ends otherwise being correctly configured. For example, configuring cpwVcID as 500 on this PE but 501 on the remote PE for what is meant to be the same pseudowire would prevent the two signaling sessions from ever correlating to the same VC, leaving the pseudowire down even though both PEs can reach each other.