When you create a Tuition record, the Pay Category is copied across from the staff member's record in the Contacts file. If the teacher is a Sessional teacher (paid for each student that they teach), then 'Sessional' will copy across. 'Sessional' records are imported into the Salaries files to calculate payments. Occasionally you may have a staff member who is SALARIED, but they teach a couple of students over and above their 'load', so they need to be paid for these students as if they were a SESSIONAL member of staff. You can change the Pay Category of individual records in the Tuition file.
As staff members loads change from year to year (and sometimes term to term), you will probably want to RESET ALL Tuition records at the end of the year - back to the default Pay Category as determined by Employment Type in the Contacts file.
Click this script to reset ALL Tuition records in one step. Be careful, though. This step can't be undone. Don't do it until the pays have been finalised for the year. And if you are a multi-campus school, be warned that this script is performed with FULL ACCESS PRIVILEGES, so it will reset ALL records for all campuses. (Perhaps check that everyone is ready before you do it!)