Classrooms prioritization

For each lesson you can define Optimal, Normal, Bad and even Emergency classrooms:

By the default the software tries to place as many lessons as possible into first two options - optimal classrooms, then it uses normal rooms.
If there is none available, it can use bad, however it still consider it as fulfilling the constraint.

Putting the lessons into emergency classroom is considered as relaxing the original constraints – so the software will do it only after it was not able to finish the timetable by using better rooms for this lesson. This case will be also included in the list of broken constraints (if you click on "Verification").

It is also possible to define these rooms priorities for subjects.
How can I specify default (usual) classrooms for certain subject?