A UNF is only needed when you need to break a connection for maintenance or installation. A screwed RSC collar is legal for a coupling. Hence my question to the author. I wasnt sure what he was asking...
Keeping the integrity of 5 threads engaged, and sealing enclosures which houses anything that arc's, and/or when changing from one classification area to another is mostly the intent of alot of haz wiring. Find a NEC Handbook and read up on it.