If your school has different bell times for different parts of the school, then the software tries to guess which bells shall be printed for each teacher. Eg if the teacher has all the lessons in one part of the school, then that bell is printed. But if the teacher is teaching in two parts, it is not clear what shall be printed.
Then you can specify for each teacher his/her bells and then these are printed:
This article was viewed 59224 times.48429 / 59224