I cancelled a session — what happens to the attendance and passes already recorded?
What matters is who cancelled. You choose that in the cancel dialog's "Who cancelled?" question, and "The student" is selected by default — so by default every affected student is charged a cancellation fee, one pass credit.
If you pick "Me" instead, nobody is charged: attendance already recorded is removed, and the sessions used come back onto students' passes. The switches under "Deduct a pass credit" let you override the choice per student, for any exception.
You can undo a cancellation at any time — every cancellation fee is returned to the passes. In your stats, a cancelled session is counted separately, not among the sessions you held.
Warning: A cancellation fee still deducts a session from a student with no valid pass — their balance goes to −1, meaning they owe one session. If you use simple payment mode, there's nothing to deduct: cancelling never charges anyone there.