In addition, several other key concepts are valuable when making a digital document system. The idea of a document ID, or handle, which does not point to a specific representation of a document, but instead references a collection of representation, is essential to deal with a document which may come in multiple forms. The use of MIME types allows easy integration into existing standards, such as the World-Wide Web. Finally, the ability of a digiment to include the content by reference instead of embedding it allows all the necessary information to browse a digiment to be sent with a very low overhead. The browser can then choose which content parts it wishes to transfer at its convenience.
Although the digiment of the future may be based on other standards, such as OpenDoc, the key concepts and ideas that went into this system will be valuable in creating a generic, expandable system for transferring and viewing digital documents.
Once we have separated the semantics of a document from its representations, we can talk about a document without having to choose a specific representation. However, to manipulate these new types of documents on-line, we need to define a new type of digital object. This new object is a digiment. The digiment contains all of the semantic information about a document, such as bibliographic information, as well as all of the representations of the document that are available. By referring to digiments, people can pass around the abstract notion of a document without having to be concerned with the representation, which is irrelevant to the information that the document is presenting.
A true document does not just consist of arbitrary data types which can be viewed in any order. Documents have notions such as "next" and "previous", which determine the order in which information should be presented. In addition, documents have the notion of "pages" and "sections". The digiment allows its contents to be structured into pages and sections, as well as preserve the "next" and "previous" relationships between the pages and sections. In addition, the digiment extends this structure to alternate versions of the same data. For example, page 12 may have greyscale and color images, while page 13 may have greyscale and color images, and a Postscript version of that page.
Furthermore, documents contain relationships between objects and sub-objects. Page 12 may have a figure embedded in it, which is also available in a separate, expanded image. A Table of Figures part could list all the figures available.
By defining structural elements that are specifically designed for documents, programs can edit and view digiments while preserving the actual structure of the document.