The buildings and the hardware
The kinds of buildings and their doors directly affect the hardware choices. Older houses often have mortise locks or thick wood doors that accept full-size deadbolts and brass knob sets; those need different cylinders and longer bolts than the modern bored doors that use tubular locks. Small multiunit buildings use keyed-alike or master-key systems, which require careful cylinder selection and key control. Storefronts and metal-frame doors use commercial-grade cylindrical or exit devices that need heavier-duty latches and sometimes dedicated panic hardware. Knowing the door structure tells you whether to replace the cylinder, install a reinforced strike, or upgrade to a commercial cylinder.