document-type Document Type

The “document” is the part of the source that is typically delivered when a user follows the link to a <related-object>. This attribute names/describes what that document is.
OPTIONAL on element: <related-object>
Value Meaning
Text, numbers, or special characters Names the type of document being returned by the Related Object Information element, for example, a chapter, appendix, or reference list.
Restriction @document-type is an optional attribute; there is no default.