Company Sergeant Major (CSM) is an appointment held by Warrant Officers Class 2 in the British Army and Royal Marines. The CSM is the senior non-commissioned officer of a company, responsible for standards and discipline. In combat, his prime responsibility is the supply of ammunition to the company.
Squadron Sergeant Major (SSM) is the equivalent in the Royal Armoured Corps, Special Air Service, Royal Engineers, Royal Corps of Signals, Army Air Corps and Royal Logistic Corps. Battery Sergeant Major (BSM) is the equivalent in the Royal Artillery and Squadron Corporal Major (SCM) in the Household Cavalry. First Sergeant would be the equivalent in the United States Army.
A CSM, BSM or SSM is generally addressed as "Sergeant Major" (or often as "Sir" or "Ma'am" by subordinates). However, a cavalry SSM is often addressed as "Mr (surname)" by officers, and as "Mr (surname), Sir" by his inferiors, with the salutation "Sergeant Major" being reserved for Staff Sergeants. Typically for the British Army, protocol can vary widely between units, creating a minefield for outsiders, new recruits, and soldiers serving outside their parent regiments.