WebUntis Substitution Planning offers various solutions for open substitutions:
The new selected teacher gets updated in time table. When the substitution gets published, teachers receive a push notification.
For each teacher you see criteria, if and why this teacher is fitting for this lesson.
See: Criteria for suggested substitutions
The lessen shows up in time table as "cancelled". So students know, that there is no class at that lesson.
This solution can be used to show that it is not necessary to have a substitution for absent teacher.
If there are more teachers in this lesson (team teaching) they will be displayed.
In case there are no further teachers in this lesson, students are to work on their own responsibility.
Using this feature you can ask teachers if they want to do a substitution. See: Asking teachers
In course of next months this list of available solutions will grow. Some of the new solutions you will already know from Untis.