Normally you would get to the document-node of the current document by calling root(.) in an XPath within the ODD. This works, but there is a caveat:
TEI Publisher will not always pass the entire document to the ODD! For example, if documents are viewed page by page, TEI Publisher will first construct a virtual TEI document containing only the relevant content of the page. This is necessary to make sure the content is well-formed XML. Calling root(.) may thus not return the original document-node but just the constructed root.
To compensate for this, TEI Publisher always passes in an external parameter pointing to the original document-node. It can be accessed via the variable $parameters?root.