Type of construction is the IBC classification system for buildings based on the fire resistance of structural elements. Type I (non-combustible, fire-resistant: typically concrete or protected steel high-rises). Type II (non-combustible, less fire resistance: typically unprotected steel commercial). Type III (exterior masonry walls, interior wood: traditional commercial). Type IV (heavy timber: large exposed wood members with inherent fire performance). Type V (wood frame: typical residential and small commercial). Each type has subtypes A and B based on whether structural elements are protected (A) or unprotected (B).
Type of construction drives the allowable height and area per IBC tables, which limit how big a building can be on a given site. Sprinklers can substantially increase allowable height and area. Mixing types (a Type I podium with Type V wood-frame above, for example) is common in mid-rise residential. Selecting the right type during early design balances cost (Type I is most expensive, Type V is least), allowable size, and code compliance. Getting this wrong forces redesign late in the project.