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A subject represents a course or class. The student can either chose it as major or minor. Multiple subjects get grouped in fields, which again can be condensed into categories.
One pair of majors and minors (subjects) is called a choice. It is always made by the student. Whereas an option describes two sets of fields or arrays of fields in which the subjects have to lie. A combination is (as the name might suggest) a special combination of the available subjects defined by the fields of the option.