(v)"commanding officer", when used in any provision of this Act, with reference to any separate portion of the regular Army or to any department thereof, means the officer whose duty it is under the regulations of the regular Army, or in the absence of any such regulations, by the custom of the service, to discharge with respect to that portion of the regular Army or that department, as the case may be, the functions of a commanding officer in regard to matters of the description referred to in that provision;
