the services and the Ministry of Defence, provides the "formal legal basis for the conduct of defence". The three constituent single-service committees (Admiralty Board, Army Board and Air Force Board) are chaired by the secretary of state for defence. The chief of the defence staff (CDS) is the senior-most officer of the armed forces and is an appointment that can be held by an admiral, air chief marshal or general. Before the practice was discontinued in the 1990s, those who were appointed to the position of CDS had been elevated to the most senior rank in their respective service. The CDS, along with the permanent under secretary, are the principal military advisers to the secretary of state. All three services have their own respective professional chiefs; the First Sea Lord for the Royal Navy, the chief of the general staff for the Army and the chief of the air staff for the Royal Air Force. Personnel The British Armed Forces are a professional force with a strength of 153,290 UK Regulars and Gurkhas, 37,420 Volunteer Reserves and 8,170 "Other Personnel" . This gives a total strength of 198,880 "UK Service Personnel". As a percentage breakdown of UK Service Personnel, 77.1% are UK Regulars and Gurkhas, 18.8% are Volunteer Reserves and 4.1% are composed of Other Personnel. In addition, all ex-Regular personnel retain a "statutory liability for service" and are liable to be recalled (under Section 52 of the Reserve Forces Act (RFA) 1996) for duty during wartime, which is known as the Regular Reserve. MoD publications since April 2013 no longer report the entire strength of the Regular Reserve, instead they only give a figure for Regular Reserves who serve under a fixed-term reserve contract. These contracts are similar in nature to those of the Volunteer Reserve. , Regular Reserves serving under a fixed-term contract numbered 44,600 personnel. The distribution of personnel between the services and categories of service on 1 April 2021 was as follows: , there were a total of 9,330 Regular service personnel stationed outside of the United Kingdom, 3,820 of those were located in Germany. 138,040 Regular service personnel were stationed in the United Kingdom, the majority located in the South East and South West of England with 37,520 and 36,790 Regular service personnel, respectively. Defence expenditure According to the International Institute for Strategic Studies and the Stockholm International Peace Research Institute, the United Kingdom has