Q. How to cite a guideline in NLM format?
While the NLM does not have a citation example for a guideline, it does state to follow the print format for a book or journal found on a Website. Of course, you'll need to indicate that it was found on the Internet.
"Book on the Internet with an organization as author having subsidiary division" seems to cover your guideline. It is what librarians call a "monograph," which is a larger catagory that includes anything from a 6 page tract to a thousand page book.