ISO 20247:2018 specifies the International Library Item Identifier (ILII), which is used for the unique identification of items held by libraries and related organizations. "Library and related organization" here refers to an organization within the scope of ISO 15511. "Items" here refer to materials identified and managed by a concerned organization.
The purpose of ILII is to facilitate unique identification of library items when information about them is shared among library applications. Examples of such system(s) include interlibrary loans and shared print agreements.