The buildings and the hardware
The building types here affect the hardware a locksmith sees. Many houses are older wood-frame or rowhouse doors with mortise lockcases recessed into the edge of the door; those require different parts and know-how than modern bored cylindrical locks. Small apartment buildings and multiunit properties often retain original heavy-duty hardware or use keyed-alike systems, which matters if you want common keys for several doors. Storefronts and ground-floor commercial units use aluminum-framed doors, rim cylinders, and exit devices that need different cutters, drills, and replacement parts than a typical residential deadbolt.