This source is a video lesson from a YouTube channel called "Irfan CFPS" that explains the different occupancy hazard classifications used in NFPA 13, the standard for the installation of sprinkler systems. The speaker describes three main hazard classifications: light hazard, ordinary hazard, and extra hazard. Each of these classifications is further broken down into subgroups based on the quantity and combustibility of the materials present in a given space and the potential rate of heat release in a fire. The video uses a variety of examples to illustrate these classifications, such as churches, bakeries, and chemical plants.