When applying vouchers, each voucher and event has its own specific conditions. Hence, there are events that do not allow the use of vouchers and there are vouchers that cannot be applied to certain events.
In any case, right before the confirmation of the purchase, there will be an informative screen showing you the total amount that your credit card will be charged, as well as the discount applied if you use a voucher.
Plus, if an event does not permit the use of vouchers, a text will appear stating that "this ticket does not admit vouchers".
For more information on the use of vouchers, click here.