text: Original usage Until the 3rd century, dux was not a formal expression of rank within the Roman military or administrative hierarchy.
  In the Roman army, a dux would be a general in charge of two or more legions. While the title of dux could refer to a consul or imperator, it usually refers to the Roman governor of the provinces.
  In writing his commentaries on the Gallic Wars, Julius Caesar uses the term only for Celtic generals, with one exception for a Roman commander who held no official rank.
  Change in usage
  By the mid-3rd century AD, it had acquired a more precise connotation defining the commander of an expeditionary force, usually made up of detachments (i.e., vexillationes) from one or more of the regular military formations. Such appointments were made to deal with specific military situations when the threat to be countered seemed beyond the capabilities of the province-based military command structure that had characterised the Roman army of the High Empire.
  From the time of Gallienus onwards for more than a century they were invariably Viri Perfectissimi, i.e., members of the second class of the equestrian order. Thus, they would have out-ranked the commanders of provincial legions, who were usually Viri Egregii – equestrians of the third class.