defense artillery, cavalry, special forces, or aviation regiment, generally wear their assigned basic branch insignia; however, as an option, Soldiers who are not affiliated with one of the above regiments, but who are assigned to a color-bearing regiment or separate operational battalion of their branch, may wear their branch insignia with the numerical designation of the battalion or regiment affixed, when approved by the Army command (ACOM) commander, Army service component command commander, or Army direct reporting unit commander. Officers and enlisted personnel assigned to cavalry regiments, cavalry squadrons, or separate cavalry troops are authorized to wear cavalry collar insignia in lieu of the branch insignia, when approved by the ACOM commander. Certain special assignment insignia is worn in place of branch insignia to denote the officer's or NCO's particular responsibility. The enlisted versions of these are borne on a golden disk in the same manner as their customary branch device. Officers so assigned continue to wear their basic branch colors on the epaulets of the blue Army Service Uniform and the Army Blue Mess uniform. Officers and NCOs serving a term in an Inspector General billet wear Inspector General insignia. Officers assigned to general staff billets wear General Staff insignia. Officers assigned as aides-de-camp wear aide-de-camp insignia which denotes the rank of the officer or official whom they serve. Nominative sergeants major will wear nominative Senior Enlisted Leader collar insignia (previously referred to as command sergeant major insignia or enlisted branch immaterial insignia) worn by command sergeants major and sergeants major when in a position rated by a general officer or senior executive service level civilian. The sergeant major assigned as the Sergeant Major of the Army wears unique insignia based upon that of an aide-de-camp to the Army Chief of Staff. If the Senior Enlisted Advisor to the Chairman is selected from the Army, that sergeant major wears unique insignia based upon that of an aide-de-camp to the Chairman of the Joint Chiefs of Staff. From 1920, warrant officers, being appointed into the Army "at large", wore the warrant officer device rather than branch insignia on their lapels and a larger warrant officer device on the service cap. Likewise, they wore brown as a branch color no matter their basic specialty. In 2004, warrant officers adopted the insignia and colors of the branch corresponding to their specialty and began wearing the officer insignia on the service cap. Aside