Substitution planners can ask teachers to substitute periods.
In this case you will receive a push notification on your phone. You can then view the substitution request in Untis Mobile and WebUntis.
In Untis Mobile you can find them under "Info Center" > "Substitution requests".
In WebUntis you find them under "Substitution planning" > "Substitution requests".
If you want to do a substitution you can click on accept. This will immediately assign the substitution to you and publish it in the timetable.
If you want to let the substitution planner know that you can't do the substitution you can press "Decline".
The substitution will get assigned to the teacher who accepts first.