Chapter II-1 covers construction related to structure, subdivision, stability, machinery and electrical installations — keeping the ship afloat and systems running after flooding/damage scenarios addressed by those rules.
Chapter II-2 covers fire protection, detection and extinction — preventing, detecting, containing, and fighting fire, plus escape.
Both are “construction” chapters, but II-1 is mainly watertight integrity/power/machinery safety, while II-2 is the fire-safety regime (often with the FSS Code).