While you specify the substitution, it can happen that you create conflicts. These are then shown with the red cross:
You can double click on each line and the software will show why it thinks there is a conflict.
You can change this substituion or you can press button Change to change the conflicting entity:
If the conflict is substitution(for example you have set teacher XY to substitute two different lessons at the same time, then the second subsitution is displayed.
If the conflict is with the original timetable lesson, for example you have send a teacher to do substituion but he has a regular lesson in the timetable, the software offers you to change this original lesson:
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