The extension would be considered external if it were on a different LAN than that of the 3CX . When a set is data filled you tell it the IP of the SIP server to register with, on an external it would be the public IP of the router 3Cx is on or the DDNS location, on an internal set it would be the local IP of 3CX.
What would be the point of forcing a set to be one or the other, outside it's own internal settings? Are people taking their phones home and trying to plug into the net there?