However, the unit of publication is getting smaller. Recent schemes allow for the identification of individual articles within publications. Increasingly we need to identify much smaller fragments of complete works, for example parts of text, images, video clips, pieces of software, etc. Identifiers are either 'dumb' or 'intelligent'. A dumb identifier has no inherent meaning and can only be resolved by looking it up in a database.
Intelligent identifiers contain some meaning.
This is a relatively intelligent identifier because its various parts have some meaning. The first part identifies the country, language or geographic region in which the book was published for example. However, as book rights are sold from one publisher to another the intelligence of the ISBN decreases and any particular ISBN can only be resolved by querying it against a central database. As the unit of publication gets smaller, the number of identifiers required grows and it becomes increasingly difficult to maintain intelligent identification schemes. The trend is likely to be towards 'dumb' identifiers.