The Primary Instance of a course is a special nomination that can be given to one instance of each course at a time. It gives the instance a special URL that has the course name slug twice instead of having the course name slug followed by the instance name slug. The main use case is to be able to get a shareable link that will always point to the most recent course instance. This way links to Lovelace courses from other sites will not become obsolete whenever a new course instance is created.