How were soldiers assigned?

[Sardaukar]

Roman army seemed to have been reluctant to have unit serve in area it was recruited (at least during Principate etc.), which is understandable, considering troops were often used as "law-enforcement".

But it's unclear to me if units continued to recruit in area where they were first formed or if they started to recruit around where they were based. I think former would make sense, maybe they sent "press-gangs" into their recruitment area and probably also had prospective volunteers arrive straight to unit location by themselves (I bet these were the guys with letter of introduction).

This theory would mean that for example legion situated in Pannonia but formed in Gaul would continue to recruit in Gaul. They'd still probably accept volunteers who would arrive to unit directly...but maybe they had something like permanent recruitment party in area they were "attached to".

Anyways, there has to be some studies and literature about this. 8)