uDoc is a MicroXML <http://dvcs.w3.org/hg/microxml/raw-file/tip/spec/microxml.html> markup language specifically designed for authoring topic-oriented documentation.
The current specification source, in uDoc, is in ud_spec_src.zip and ud_spec_lib.zip. The OmniHelp version is in ud_spec_omnihelp.zip; unzip in its own directory, and double-click _ud_spec.htm. Or download the Word doc in ud_spec_doc.zip.
Related repositories here are for the MicroXML parser at <https://github.com/jeremygriffith/mxl-MicroXML-parser>, for the udx utility at <https://github.com/jeremygriffith/uDoc-UDX-Utility>, and for OmniHelp at <https://github.com/jeremygriffith/OmniHelp>.