From what I read in this chapter he explained the means of egress which is  going out or leaving a place, as well as stair design which includes the different types of stairs and the reasons behind handrails, treads, risers, their dimensions and etc. Every building has an occupant load and that is the number of people that may occupy a building or portion. This plays a role when it comes to building stairs because these passageways should be as direct as possible and obtain safe egress. Exits are also very important because an exit must provide an enclosed and protected means of evacuation for the occupants of a building in the event of fire. In the section where he speaks about stair design he explains that the dimensions of risers and treads in a stairway should be proportioned to accommodate our body movement, also the occupant load which is based on the use group and the floor area served, determines the required width of an exit stairway. As you can see in this chapter the reasoning behind the way stairs, exits, and the specific dimensions chosen are made because of building codes which accommodate us while either leaving the building going in or going from different floors.