This IpAddress is 'the address as known in the outside world' per its description, the per-virtual-interface counterpart of the general static NAT global address. An admin binds a static mapping to a specific VI (rather than the whole system) when a device has multiple external-facing interfaces and the published address must be reachable only through one of them, such as a DMZ interface. For example, an admin publishing an FTP server reachable only via the DSL uplink VI would set this to the public address assigned on that specific interface.