Subject is the basic building block for a course. A course can be made using one or multiple subjects.
Subject consists of subject matter. It includes structured study material contents that learners will study, assignments or tasks or projects that learners will work on and submit for teacher's review, online tests that learners will take either for practice or for examination, practice exercises that learners will use for extra practice etc. One or multiple subjects can be used to make a course. Learners enroll in courses.