Because time and budget are fixed, the only remaining variables are the requirements. So if a project is running out of time or money the requirements with the lowest priority are omitted. This does not mean that an unfinished product is delivered, because of the pareto principle that 80% of the project comes from 20% of the system requirements, so as long as those most important 20% of requirements are implemented into the system, the system therefore meets the business needs and that no system is built perfectly in the first try.