A digital item is a computer based file of data holding a picture, drawing, photograph, a sound or film recording, an image of text, or a software application. A digital item in a "library" collection need not be physically present in the library. It could exist anywhere in the world, the library and their users can access it provided they have the authority, access and the capabilities of playing or rendering the item. SLIM21 provides methods by which libraries can describe such items to their users anywhere in the world.

Authorized users of a library can submit such items for inclusion in the library catalogue. They submit such items through a web-based service provided by SLIM21. Another set of authorized users can communicate with the library on the subject of quality, usefulness, appropriateness of such submitted items. This community participation in the inclusion of the item is found very useful especially in the intranet scenarios where SLIM is employed to build a collection of digital items in an enterprise.

When the library decides to place such an item in the catalogue, the metadata is carefully prepared and the item is added to the catalogue. SLIM21 provides for inclusion or exclusion of metadata elements that are used in the creation of the catalogue entry. SLIM21 also stores records of such data entry formats in its database. The cataloguing of digital items is done using exactly what the library chooses. SLIM21 also provides initial versions of data entry formats for various types of items. Libraries can build their own formats by modifying these. SLIM21 covers, and extends if required, the popular Dublin Core format.

One crucial data element in the description of digital items is the unique address of the digital item, its URI. SLIM21 presents this URI as a hyperlink whenever the item description is presented to the user's browser page. Users only click on the link to begin accessing the item. It is possible in appropriate versions of SLIM to allow the view of digital items to only the authorized users. It is also possible, when it is otherwise permitted, to have a "local" copy of an item cached in the library collection.