The + substitute
You can edit a substitution without assigning a substitute teacher. Enter "+" in the field "Substitute" and the substitution is marked as edited (It is not an open substitione any more). You can enter clarifications in the substitution text.
Contrary to absence
If an absent teacher shall - as an exception - hold his lessons (e.g. an examination) then you can assign the teacher after answering the query "Schedule anyway?" with <Yes>.