A document is a piece of paper, a booklet, etc.; providing information esp. of an official nature. In TIMe Documents are physical carriers of information. This information may be local to that document, or it may be fetched from descriptions and models (whole or partial models). Documents are often made for specific occasions and audiences, e.g. a contract, a review document, a user manual.
A description or model may appear in several documents, therefore the descriptions or models should be maintained separately from the documents.
A document may be seen as a "snapshot" at a particular point in time. As such it need not be maintained, although it may be.