Membership of the Armed Forces
Letters denoting membership of one of the armed forces come last in the order of precedence of letters after the name, and follow all other letters: Orders and Decorations conferred by the Crown, Royal Appointments, other Appointments (Queen's Counsel, Justice of the Peace and Deputy Lieutenant) and university degrees. All letters after the name are preceded by the abbreviation Bt (Baronet) - if applicable - and Esq.
The letters RN (or Royal Navy, which this Service prefers) are placed after the names of serving officers of the rank of Captain and below. They are also placed after the names of retired Captains, Commanders and Lieutenant Commanders. The letters RNR are likewise used by officers of the Royal Naval Reserve
The appropriate letters which signify a Regiment or Corps may be placed after the name for officers on the active list of and below the rank of Lieutenant Colonel, but are often omitted in social correspondence. These letters are not used for retired officers.
Corps have letter abbreviations (for example RE for Royal Engineers, RAMC for Royal Army Medical Corps, RAOC for Royal Army Ordnance Corps, REME for Royal Electrical and Mechanical Engineers). Most regiments are written in full.
Royal Air Force
The letters RAF are placed after serving and retired officers, except for Marshals of The Royal Air Force. Officers above the rank of Group Captain do not often use these letters. The same rules apply to the Royal Air Force Volunteer Reserve (RAFVR).
The letters RM (or Royal Marines, which some officers prefer) are placed after the names of serving and retired officers of the rank of Lieutenant Colonel and below. The same rules apply to the Royal Marines Reserve (RMR).