Legions of the Severus scenario

[Vincentius]

The first image of a legionary cohort below is from the Severus scenario, under it there is a write that says "empire_legion" ; looking at the other images of legionaries cohorts in the units folder of the game I noted that some of them are "Mid_Empire" (e.g. second image). Well, my question is, what differences are there between "empire" and "mid_empire" images ? Depending on what you have put the "empire" legionary images instead of "mid" ones in the Severus scenario and also in the "Parthian Severus scenario" ?