Identifies which version of the DTD, named in the DOCTYPE declaration, was used to make the article under consideration, which therefore can be used to validate this article. (Authoring Note: This is a fixed value (meaning that the value is set in the DTD and an author or editor cannot change it) that should change every time the DTD changes versions or revisions.)
This is a “fixed” attribute, with the entire attribute definition defined in a Parameter Entity within the DTD; thus, the Parameter Entity can be changed to reflect a later version of the DTD.