Use 'commissary' when talking about military or prison retail services, signifying a specific type of store. She was heading to the commissary to pick up supplies for her dorm.
Pro Tip 2/3
Not Public
'Commissary' often refers to a private store, not usually accessible by the general public. Visitors were not allowed to purchase items from the base commissary.
Pro Tip 3/3
Prison Use
In a prison, 'commissary' is more than just a store, it's often the only place inmates can buy personal items. He saved his earnings to buy snacks from the prison commissary.