BGP connector attribute is a transitive attribute (unchanged by intermediate BGP speaker node) that is carried with VPNv4 advertisements. It specifies the address of source PE node that originated the VPNv4 advertisement.
With Inter-AS MVPN Option B, BGP next-hop is modified by local and remote ASBR during re-advertisement of VPNv4 routes. On BGP next-hop change, information regarding the originator of prefix is lost as the advertisement reaches the receiver PE node.
BGP connector attribute allows source PE address information to be available to receiver PE, so that a receiver PE is able to associate VPNv4 advertisement to the corresponding source PE.