text: Duces differed from praesides who were the supreme civil as well as military authority within their provinces in that the function of the former was purely military. However, the military authority of a dux was not necessarily confined to a single province and they do not seem to have been subject to the authority of the governor of the province in which they happened to be operating. It was not until the end of the 3rd century that the term dux emerged as a regular military rank held by a senior officer of limitanei – i.e. frontier troops as opposed those attached to an Imperial field-army (comitatenses) – with a defined geographic area of responsibility.
  Diocletian's reforms
  Under Diocletion, during the Tetrarchy, a new office called dux was created with powers split from the role of the governor of a province. The dux was the highest military office within the province and commanded the legions, but the governor had to authorise the use of his powers after which the dux could act independently and handle all military matters. The Dux Belgicae secundae ("commander of the second Belgic province") is an example.
  Also the provinces were reorganised into dioceses with each diocese administered by a vicarius. As with the governors, the vicarius was assisted by a dux. This dux was superior to all other duces within the dioceses; when the vicarius called the legions of the dioceses into action, all of the legions were at the command of the dux. The office of dux was, in turn, made subject to the magister militum of his respective praetorian prefecture, and above him to the emperor. The Dux per Gallias of the diocese of Gaul is an example of this office.