There are many books and internet materials which treat these as cohesive units. Perhaps it's also appropriate to show the nearest cognates with other classification systems. One might envision other ways to break up the discipline and see how even they are. Occasionally however we find topics which are arguably cohesive branches of mathematics, but which have over the last few decades not been accorded an MSC heading of their own. There are some subjects which, based on reports in the popular press and the presentations in the undergraduate curriculum, one might expect to be major portions of the subject classification system.
Elementary calculus is considered in Real functions .
Topics in Approximations and expansions and Sequences and series are usually also included in calculus courses. Mathematics is frequently presented to target audiences, with the theory selected, and the examples configured, to match the practical needs of the audience. In other cases, some more or less sophisticated mathematics is used, gathered together irrespective of the mathematical basis of the tools. It's always possible, perhaps, that a person lands at this web site completely by mistake.