If you want to add metadata to a given document, add it to the table within the Document Properties Marker Macro at the top of the page. This is a simple name/value list with an additional column that contains row controls.

While a document type requires a certain set of properties to be present, authors may add any property they require. But keep in mind that it is difficult to remember properties that are not standard. Further more it is sometimes very difficult to sense the meaning of document properties that are not documented.