The Plumber should be one of the heroes of modern society. In the 19th century, people moved closer together for work in Industrial Revolution jobs. Typhoid, cholera and dysentery killed thousands due to issues of human waste. Modern plumbing methods, and our Hero the Plumber, helped eliminate this means of spreading infectious diseases. Look at societies today with poor sanitary sewer systems and you'll see a high infant mortality rate and a low longevity.
Building Codes recognize this connection between adequately designed and installed sanitary plumbing systems and the public health. To design sanitary systems, most Codes rely on a clever concept called Drainage Fixture Units (DFUs). By setting a DFU quantity for each type of plumbing fixture, the Code considers both how much water a fixture typically passes and the probability of how often the fixtures will be used. In a four story office building with a common toilet rooms on each floor, for example, all the toilets, lavatories, urinals and sinks won't get used at the same moment. If plumbing systems had to be designed for that scenario, pipe sizes would be huge.
The DFU concept allows reasonable sizing of sanitary sewer lines, based on experience from many buildings. Of course, the Plumbing Engineer may decide to design more conservatively than the DFU requirements. A football stadium, where all the beer gets drained from bladders during halftime, needs to be designed for all fixtures to flow at one time. Generally, though, the DFU concept gets used often and the basics should be understood by the Construction Supervisor.