Once you have defined departments you can assign them to classes. For this, display the department columns in the classes master data.

Then assign the relevant department to the classes via this column.

By assigning the departments to the classes you implicitly define which departments lessons are assigned to. The “Department” column can be displayed in the lessons windows . This is, however, purely informational as the data cannot be edited here.
According to this example all lessons involving classes 1a or 1b have now been assigned to departments.

The following rule applies: a lesson has the department of the class assigned to it. If more than one class is involved in a lesson, the first class in the sequence determines the department.
As lessons 11, 7 and 73 in the above example involve a coupling of classes 1a and 1b, and as 1a is the first class entered, the lesson belongs to the department of class 1a.