When you work according to the "Teacher-Room" prinicple which means that the students go to the teacher into the room where the lesson will take place and not vice versa, you can also use this for substitution planning. Adjust the respective settings for this: go to "Cover scheduling | Miscellaneous".
A pre-requisite for this principle is that you have defined rooms in the master data of the teachers.
Now enter a teacher for substitution and the room of this teacher is automatically also assigned to this substitution, provided the room is available.
If the room of the teacher is not available due to another lesson taking place there, you will be asked what you want to do next.