The value of custMultSvcSiteSubscriberMss specifies whether
this multi-service site will be used as the default multi-service site for
subscribers linked to the port specified in custMultSvcSiteAssignment when
the use-subscriber-mss flag is set on the subscriber's subscriber-profile.
This field can only be set when the value of custMultSvcSiteScope is set
to 'port(1)'.