perhaps instead of asking such a question you were to just tell us what your situation is that you need an answer for and someone would be able to help you.
If the spec says it has to be in rigid conduit, then it would appear enclosures are not permitted at all. However, I would suggest that all manner of conduit fittings would be allowed since they are part of the RGS system unless they are listed in such a way as they could not be used in the environment they would be installed in.