The system bills students on the first day of the month (for existing students) or at the onset of the second class (for new students). The amount billed is deemed as a credit that the student slowly utilizes as the month progresses.

Students are charged for each class, so a class attended is equal to credit usage of the equivalent amount. So, if a student reduces the # of classes somewhere along the month, then he/she has "overpaid" for the month and hence receives credit equivalent to the # of classes reduced. 

The credit is made available for any future usage; usually applied to next month's billing amount.