Course A }
Course B } <--- All link to ---> Course D
Course C }
Course A maybe Geometry, Course B is Algebra, Course C is Statistics
All of these can be linked to a common Course - Maths using Course meta links to automatically enrol the students.
For example if all the students in Course D do all the 'sub-subjects' then enrol the students on Course D and have a meta link that enrols them all onto each of A, B and C. If A,B and C are options that can be taken then the initial enrolment would be onto those options and they all link to automatically cascade that enrolment into the overarching course D.
It's then upto you as a management issue of whether your teaching materials, forums, discussions and so on, from part of the overarching course or the sub-courses.
Of course part of this really depends on just how much content you have - is it beneficial to split it across multiple courses, or is it better learning design to keep it in a single course, organised using the Lesson module, or Books, etc.
Hope that helps