You can set some prerequisites that must be met by the participants. These prerequisites are displayed on the course page or the public page when a user wants to enroll in the course.
With the "Certain courses must be worked through first" feature, you simply select some courses that the user must have completed at least once before they can enroll in your course. In this setting, selected courses must have been worked through at least once before that course can be started.
On the other hand, you can also exclude some courses that should not be started simply by adding them in the same option. In this case, access to the courses will be blocked.
Use this exclusionary feature if you want to sell variants or difficulty levels of similar or same course content, e.g. an "amateur" and a "professional" course. In this case, logged-in teachr user receive a notice that registration is not possible because they have already registered for the courses you selected in this setting.
Excluding courses only works for users who already have a teachr account that has statistics collected about your excluded courses. You can bypass this course by simply creating another teachr account. Please make sure that you use this feature only for unimportant cases for this reason.