This object specifies the source IP address expected on incoming MPLS frames for this static decap tunnel, and as the description states, it cannot be modified once the tunnel entry is created. Because this is the egress side, this address identifies the expected remote ingress PE that traffic must originate from for it to correctly match this decap entry -- a correctly labeled packet arriving from an unexpected source simply won't be matched here. For example, a source address of 10.0.5.1 dedicates this decap tunnel to accepting traffic specifically from that one remote ingress PE, and traffic labeled the same way but sourced elsewhere would not match this row.