Session-agent defined with no ip-address

For the Session-agent, what is the use of Ip-address and port information in a session-agent ?

I am reviewing some SBC configuration, and came across the following. I am not sure if these are correct ?

1. Session-agent defined with no ip-address

session-agent
        hostname                                xs-1-usf-us-west
        ip-address
        port                                    5060

2. Session-agent with no ip-address and port 0 defined

session-agent
        hostname                                xs-1-usg-us-west
        ip-address
        port                                    0

Any pointers are much appreciated