The buildings and the hardware
The housing stock here means a range of hardware. Older single-family homes and some duplexes often have solid-wood doors with mortise locks or older rim cylinders, which require different parts and a different approach than the modern bored deadbolt and latch preparations common in newer construction and townhouses. Apartments and smaller multiunit buildings tend to use cylindrical locks and keyed-alike systems; that affects whether rekeying or cylinder replacement is the right fix. Storefronts and small offices sometimes have exit devices or aluminum storefront locks that need specialist heads or bar alignments. Knowing the door construction and lock type before work starts saves time and avoids unnecessary cutoffs.