I take it you have read Wiki......
- The RFA counts an Aviation Training ship/Hospital Ship and landing vessels amongst its assets. RFA personnel are employees of the Ministry of Defence, and since 2003, special members of the Royal Naval Reserve deemed sponsored reserves, which are civilians who must be part of the Armed Forces in some capacity, in order to carry out specialist civilian jobs in a military capacity. Although RFA officers wear Merchant Navy rank insignia with naval uniforms, they are classed as a part of the naval service and are under naval discipline when the vessel is engaged on warlike operations. RFA vessels are commanded and crewed by these officers and ratings, augmented with regular and reserve Royal Navy personnel to perform specialised military functions such as operating and maintaining helicopters or providing hospital facilities. Royal Navy personnel are also needed to operate certain weapons, such as the Phalanx, however other weapons (such as the Bushmaster 30mm cannon, Oerlikon 20mm cannon, GPMG and the 7.62 minigun) are operated by RFA personnel.
Before the RFA contract was amended to require signatories to become members of the Sponsored List of the Royal Naval Reserves they were treated as Civil Servants under the armed forces discipline act ie in a war zone they were subject to it, outside they were not. As a sponsored reservist you can be activated outside the war zone and so become subject to the Armed Forces Discipline act. The rest hasn't changed much, the RFA is still responsible for all training, equipage, and deployment of its personnel (with the caveat of that we are still part of the naval service and under its operational control, so if the admirals say go, we go).
All the rules and regs of being sponsored a sponsored reservist are available via Google if you are bored enough to know more (I am fairly certain I have posted links to the docs on here before as well)