Authorities in most countries currently use different identification numbers to identify individuals. A national ID number is used to identify people in Norway. Foreigners who do not live in Norway will instead be given an individual D number. The tax authorities in other countries use identification numbers to identify people in the same way. You must state your foreign identification number so that the tax authorities in the place where you are tax resident can link the account information to you.
Identification numbers may either be used by the tax authorities alone, or by the tax authorities and other agencies and institutions in the country concerned, as is the case in Norway. You will normally find your identification number on documents sent to you by the tax authorities in your country of residence, but the number may also be given on passports, national identity cards, driving licences and other official documents.
The tax authorities in Sweden, for example, use a personnummer, Denmark uses a CPR number and Poland uses a PESEL number as an identification number.
Such identification numbers are often referred to as a TIN ("Taxpayer Identification Number"). Not all countries use such identification numbers. More information regarding the identification numbers used in various countries can be found here.