In certain types of schools, student groups rotate over the year.
A class is split into 4 groups. They have four different teachers teaching a certain subject in four different workshops. The subject stays the same for the entire year, only the student groups rotate every 4 weeks.
If you want to know how to correctly reflect this constellation in Untis so that the correct students are in the class register in WebUntis, please read this article: "Student group roster".