With a national identity number you can for example open a bank account, get a regular GP (fastlege), rent a place to live, pay your taxes and be able to vote at elections.
A national identity number is an eleven digit ID number, and the last five digits are called a "personal number".
There may be several reasons why you have been registered as moved from Norway in the National Registry.
If you are registered as moved from Norway because your residence permit is no longer valid and you have a new application that is being processed now, you may ask the Norwegian Directorate of Immigration (UDI) to notify the Norwegian Tax Administration, so that they correct the registered relocation. This may only happen if the Norwegian Directorate of Immigration believes you were legally in Norway when you were registered as moved from Norway.
If there are other reasons why you are not registered in the National Registry any more, you need to contact your local Norwegian Tax office. (external website)
Your national identity number is not necessarily the same as your birth date. The Norwegian Directorate of Immigration does not change your birth date when it’s not the same as your national identity number.
Everyone who applies for protection (asylum) or for a residence permit in Norway will be assigned a DUF number. The DUF number is your registration number in the computer system of the Norwegian Directorate of Immigration (the UDI).