How do I know which credit note relates to which invoice?

On your credit note there will be a line on the top right called "Order Number" , this corresponds directly with the original order number that this was placed under. In fact, all the information on the top right (excluding Cr. Memo No and the Date) is the original data to help you trace this back.

Please note - if the order number does not correspond with any order numbers on your platform, it may be a case that the voucher has been reissued (vouchers expire after 30 months for auditing purposes/to make sure the funds are not being used for children that are too old, and are reissued upon the employee’s request) If this is the case, please contact Customer Care by clicking "Can't find what you're looking for" and they will be able to trace the original order number for you.