A staff clerk is a professional specialist clerk in the British Army who is a member of an administrative corps, as opposed to a unit clerk, who is a member of the corps or regiment in which he or she works and is trained first and foremost in the duties of that unit (as an infantryman, for instance).
Staff clerks used to predominantly work in headquarters units alongside staff officers, hence the name. Today, with the increasing complexity of administrative systems, they are more widely spread and are employed in most units. They used to belong to the Royal Army Service Corps until its disbandment in 1965, when they transferred to the Royal Army Ordnance Corps. In 1993 they again transferred, to the Staff and Personnel Support Branch of the Adjutant General's Corps.