An area where stacks are stored. A workman will carry newly arrived stuff from the Delivery zone to the storage room. Unused materials and dismantled objects will be stored here. If there is no Storage area, objects will be left on the floor where they drop.
Make a storage room near the kitchen and/or central area to minimize walking distance.
Place a metal detector at the door(s) of the Storage so that Prisoners who are snooping around and decide to steal metal Contraband from the Storage are caught.
Better yet, one may assign the Storage room as "Staff Only" in the Deployment tab, so that working and curious Prisoners do not walk in and steal Contraband.
There's no need for this room to be indoors (or outdoors) explicitly. As there are no required objects and no automatic purchases of items related to this room, it may easily be temporarily set just about anywhere, e.g. during construction phase of the prison.