This is the local SCTP port number this specific association is using, which - combined with the remote port and address - uniquely identifies the association on the wire, distinct from a TCP port even though SCTP ports are drawn from a similar numbering space. It's the value to check when multiple applications on the same device use SCTP and an engineer needs to confirm which local service owns a given association. Seeing an association using local port 2905 immediately tells an admin it belongs to SS7-over-IP signaling (M3UA) rather than some other SCTP-based application on the box.