Construction type is how the IBC describes what a building is made of and how well its structure resists fire. Every set has one, and it drives allowable height and area, required fire-resistance ratings, and which materials are permitted.
The IBC defines five types, I through V (IBC Chapter 6, Tables 601 and 602). Type I is the most fire-resistant — noncombustible structural frame with the highest ratings, used for high-rises. Types II and IV are noncombustible or heavy-timber. Type V is the least restrictive, typically wood-framed. Each type is further split into A and B: the A subtype carries higher fire-resistance ratings than B. Table 601 sets the required hourly ratings for structural frame, bearing walls, floors, and roofs; Table 602 sets ratings for exterior walls based on fire separation distance.
Type is not chosen freely — it has to support the building's proposed height and area for its occupancy (see the allowable height and area guide). A common mistake is declaring a construction type on the cover sheet that the structural drawings don't actually deliver, or that won't support the area without a sprinkler increase. Reviewers cross-check the declared type against Table 601 ratings and the height-and-area tables, so the cover, the code sheet, and the structural set all have to agree.
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.