ISO 15511:2011 specifies the International Standard identifier for libraries and related organizations (ISIL), which comprises a set of standard identifiers used for the unique identification of libraries, archives, museums and related organizations with a minimum impact on already existing systems.

An ISIL identifies an organization, i.e. a library, an archive, a museum or a related organization, or one of its subordinate units, which is responsible for an action or service in an informational environment (e.g. creation of machine-readable information).

Since ISO 15511:2011 allows the use of existing codes to be incorporated into the ISIL, it is possible that a given organization can have more than one ISIL. However, it is the intention of ISO 15511:2011 to minimize the number of codes.