It is a traditional badge 'Staff Officer's' badge going back to pre-world war one. In the days when Brigadiers were not considered to be 'Generals' they wore the same badge. It is worn today by Colonels who are not on the staff of a regiment. This means that most Colonels wear it but Colonels of Artillery, Honourary Colonels, Chaplains, Doctors and others, who are still on the strength of a regiment or corps, may continue to wear thier distinctive badge.