Occupancy classification is the first decision on any set, and the one almost everything else depends on. IBC Chapter 3 sorts every space by how it's used into groups: Assembly (A), Business (B), Educational (E), Factory (F), High-hazard (H), Institutional (I), Mercantile (M), Residential (R), Storage (S), and Utility (U) — most with subgroups, like A-1 through A-5.
The group you assign drives the occupant load factor, egress requirements, sprinkler and alarm thresholds, allowable height and area, plumbing fixture counts, and required separations. Get it wrong and every one of those downstream numbers is wrong too. A single building often contains several occupancies at once, each classified on its own.
A common mistake is under-classifying a space — calling a training or break room "Business" when its use and occupant load actually make it Assembly, which then understates egress and fixtures. Reviewers check that the classification on the code sheet matches the actual use shown on the floor plans, so the label, the layout, and the calculations all have to tell the same story.
This guide describes the model code for general understanding and is not a substitute for the adopted code and amendments enforced by your local authority having jurisdiction. Verify all figures against your jurisdiction's codes of record.