A course is an (Untis) lesson that need not be taken by all students in the class(es) allocated, but can be chosen by individual students.
In order to convert lessons into courses, the desired lessons must be selected by highlighting them with the mouse and then clicking on the <Convert lesson(s) into course(s)> in the toolbar of a lessons view.

A (c) is displayed in the "Cl,Te" column of a lessons view to indicate that a lesson has been converted into a course.
Warning: Unique subject name
If there are several courses allowed for a class, each of these courses should be given a unique subject name in order to make them easier to identify. For this reason, the course demo file contains e.g. two lessons for year 12 with the subject names bio1 and bio2 instead of both lessons having the subject name bio.