Class XY contains a window

The software calculates when the class MUST have lessons and when it CAN have lessons.

By default every student must have a lessons in the MUST section. So the software alerts if there is a group of students that doesn't have lesson in the MUST section. E.g. Monday 4th on the picture:

A window can be created also in the CAN section. If the CAN section is 5-7 and the class has lesson on 7th but not on 6th then 6th is window in the students timetable.

You can change the MUST and CAN have section by changing the education block manually and allowing students to come later or leave earlier. See: Education block - allow some classes to come later or to leave school earlier.