RFA is Merchant Navy (MN) is manned by civilians acting in full time support of the Royal Navy. The MN is civilian and as the name "Merchant" implies, often carries trade or cargo goods. Part of the RN's job is to protect trade routes.
The entry standards for RN are different to those for MN and in many respects the RN entry standards are more stringent but a failure to meet RN standards would not necessarily rule out joining the MN (or RFA).
However, the RFA makes up part of the Naval Service whereas the Merchant Navy does not. The RFA sits somewhere between the RN and the MN.
For service in the RFA you need an unrestricted ENG1 (seafarers medical) which means you can sail anywhere in the world whereas in the MN there are jobs that you could do with restrictions on your medical.
You are not HM Forces in the RFA, you're a crown servant - a civilian. A civilian with dog tags and a MOD 90 whose ships have guns and go to war zones. It's not for everyone.
I'm not medically fit for the RN but I am for the RFA. It's a happy compromise for me. Good luck with your application and think positively.
RFA, so far as I'm aware are not technically Civil Servants either in some respects. If they were, ex-RN personnel in receipt of a pension would have their pension abated if they joined the RFA. They don't.
Royal Fleet Auxiliary (RFA) seafarers are Ministry of Defence civilian employees with pay andterms and conditions of service that reflect their core function as seafarers.
However, because of the unique nature of the work of the RFA the organisation is viewed as a ‘Non Standard Occupational Group’ whose terms and conditions of service vary from those covering ‘main stream’ MoD civil servants.