In most provinces of the empire, commoners are born as subjects to the noble house. The particular house that they "belong" to is dependent on the citizenship of the parents or sometimes the place of birth. Subjects fall under the authority of which ever lord in currently in command of the noble house that they are affiliated with. They may only be released from the rule of their house by the head of the house directly.
The ruling house has the authority to conscript their subjects to serve in the Imperial Legion as enlisted personnel for their military beneficiaries. While this is typically done during wartime, it really is dependent on need. Any commoners who have achieved officer status within the imperial military are released from oversight by their overlord while they actively serve and may pursue their career unfettered.
Any officer or enlisted crewman who is discharged from military service is obligated to return to the seat of their ruling house to seek purpose from their lord. Any dereliction of this duty is punishable by the Prefaectus Order.
Any commoner who joins the Prefaectus Order is released permanently for their subjectivity.