If I have a parent and child Business Unit and set my child BU has "Subscribers will be unsubscribed from this business unit only" set. If the Subscriber exists in the parent BU, does the parent BU subscriber preference supersede their value in the child BU? (which are are doing the send from)
If your Parent BU is set for "subscribers will be unsubscribed from all business units in the enterprise" then its setting will supersede the preference setting of any child BU under it.
If a BU is set as "Subscribers will be unsubscribed from this business unit only" and the customer unsubscribes from an email sent from that child BU, then they will be unsubscribed from that BU. They will remain subscribed in other business units