Internal SIP line is used to bridge LCR and internal SIP proxy. It's parameters are defined in the default settings when you get the 2N® VoiceBlue Enterprise. Related to the internal line is a user, that is also created in the factory default settings too. In firmware version 2-2-8 or newer, SIP internal line doesn't request an accompanied user.
You don't need to use SIP inernal line if you don't use internal SIP proxy.