Hello, welcome to the RDM forums!
There are no licensing requirements related to the RDM protocol itself (ANSI E1.20). You are free to market and distribute anything you develop as you see fit, no documentation or limitations apply.
While there is no licensing requirements related to RDM itself, it is always possible that individuals or companies may have patented specific implementation embodiments or features that utilize RDM but there is no such thing as any kind of patent pool you have to license such as other technologies (i.e. MPEG) in order to implement and sell products using RDM. I am also not personally aware of any patents that would broadly impact RDM implementation.
A good resource for doing any research on lighting industry patents is:
https://www.mikewoodconsulting.com/p...nts_search.php
Hope this helps answer your question.