ICANN has released internet draft version 00 of the mark and signed mark definitions, also known as SMD. Follow the link to download the complete file.
The draft defines the mapping for marks and signed mark data. The SMD files generated by the TMCH will follow this definition. This draft could be referenced by EPP extensions related to the implementation of the TMCH by Registries and Registrars.
The draft has gone trough an extensive review and can be considered a stable reference.