The IAB has published RFC 7749: The “xml2rfc” Version 2 Vocabulary.

Abstract: This document defines the “xml2rfc” version 2 vocabulary: an XML-based language used for writing RFCs and Internet-Drafts.  Version 2 represents the state of the vocabulary (as implemented by several tools and as used by the RFC Editor) around 2014.  This document obsoletes RFC 2629.