A postage stamp issued by a postal authority is allocated a WNS number on the basis of the following four specific criteria:
- face value or indication of tariff;
When one of the above criteria differs on another postage stamp, the stamp is given a different WNS number.
The date of issue and type of presentation (sheet, booklet, miniature sheet, etc.) are not taken into account when WNS numbers are assigned.
The WNS number comprises the ISO 3166 Alpha-2 country code (2 letters), a serial number (3 figures) and the year of issue (2 figures), i.e. a total of 8 characters including a dot.