ISO 15511:2009 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). It can be used to identify the originator or holder of a resource (e.g. library material or collection in an archive). The ISIL is intended for use by libraries, archives, museums and agencies doing business or interacting with these organizations (e.g. suppliers, publishers, and government institutions). An ISIL identifies an organization or one of its subordinate units, throughout its life. In some cases, such as when an organization has undergone a significant administrative change (e.g. a merger with another organization), particularly one that results in a name change, a new ISIL identifier can be allocated.

Since ISO 15511:2009 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, the intent of ISO 15511:2009 is to minimize the number of codes.

Any library, archive, museum or related organization, administrative unit or subordinate unit, acting autonomously, can be allocated an ISIL.

An ISIL is not intended to be used to classify organizations or their services and holdings.