As a rule the ISBNs identify finite, manageably-sized objects, but these objects may still be large enough that resolution into a hierarchical system is appropriate. The materials identified by an ISBN may exist only in printed or other physical form, not electronically. The best that a resolver will be able to offer in this case is bibliographic data from a national bibliography database, including information about where the physical resource is stored in the national library's holdings. The group and publisher number assignments are managed in such a way that the hyphens are not needed to parse the ISBN unambiguously into its constituent parts. However, the ISBN is normally transmitted and displayed with hyphens to make it easy for human beings to recognize these parts without having to make reference to or have knowledge of the number assignments for group and publisher identifiers.
Groups usually cover only one country, but occasionally a single group is used in several countries.
For each international group, the International ISBN Agency has assigned ranges of publisher identifiers to individual countries. There are plans to extend the ISBN into 13 digits in order to make the system more suitable for identification of electronic monographs. So called Bookland ISBN will consist of a traditional ISBN preceded by the 978 or 979 EAN flag. For the ISBN namespace, some additional equivalence rules are appropriate. Prior to comparing two ISBN URNs for equivalence, it is appropriate to remove all hyphens, and to convert any occurrences of the letter X to upper case.