22 SAS Regiment being the Regular unit with 21 SAS Regiment (Artists Rifles) and 23 SAS Regiment as the TA reserve units.
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The Territorial Army (TA) is a part of the British Army, the land armed forces of the United Kingdom, and composed mostly of part-time soldiers paid at the same rate
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Selection is reputed to be the most demanding military training course in the British Army with a reported pass rate of less than 10%. It is a test of strength, endurance, and resolve over the Brecon Beacons and Elan Valley in Wales, and in the jungle of Brunei, taking around 6 months to complete.
Selection is held twice a year regardless of conditions. A candidate must be male and have been a regular member of the Armed Forces for at least three years or a member of 21 SAS or 23 SAS (which can be joined directly from civilian life) for at least 18 months. All soldiers who apply must have at least 39 months of military service remaining and to be eligible for selection the candidate must not exceed 32 years of age. A candidate who fails any stage of the selection is 'Returned to [his parent] Unit' (RTU'd). Candidates are allowed only two attempts at selection, after which they may never reapply. Many are not even allowed that.