The registration group or identifier group is the second element in a 13-digit ISBN (first element in a 10-digit ISBN) and indicates the country, geographic region, or language area where a book was published.
[1] The element ranges from one to five numerical digits.
[2] The following registration groups are compatible with or without a 978- prefix: The following must have a 979- prefix: Shorter registration group numbers are generally used for countries or regions with greater publishing volume.
[citation needed] Because a longer number leaves room for fewer publishers and ISBNs, several countries have more than one number assigned.
On the other hand, some countries (Australia, Switzerland, Fiji) have no unique number because they fall in a broader geographic region or language area.