They describe relevant aspects of the document as a whole. Some metadata are bibliographic in nature. Bibliographic metadata have a long tradition in library science. The Dublin core is an attempt to standardise the tags for bibliographic metadata. Some metadata refer to the state of a document in the life cycle.
Some metadata describe aspects of the content of the document as a whole.
The purpose of adding such metadata to documents is to allow to clearly characterise what distinguishes a specific document from related but different ones. It should go without saying that in order to be able to differentiate we need enough features or dimensions of possible differences. On the other hand in order to be usable the set of features should be limited enough. This does not mean that it has to deal with a topic exhaustively and in all detail, but that it can be read and understood without the need for other documents. Sometimes document may be comprised of subdocuments that might exist on their own.