[PJSIP] What's the relevance of "contact_user" for ITSP?

pjsip.conf.sample says

[quote]“contact_user=” sets the SIP contact header’s user portion of the SIP URI
this will affect the extension reached in dialplan when the far end calls you at this registration. The default is ‘s’.
[/quote]This doesn’t mean a lot to me. What’s the relevance of ‘s’?

The only other info I can find about contact_user is Asterisk 13 Configuration_res_pjsip_outbound_registration which says

contact_user. String. Contact User to use in request”

Again, not a huge amount there to go on. Can anyone explain in more detail its relevance to routing an incoming call from an ITSP? Thanks.

In Asterisk terms, Contact User is the extension to which incoming calls should be addressed by the peer. Some ITSPs may ignore this and send direct in dialing digits, instead.