Voucher and gift certificate payments have been included on the Money Totals by Payment Method report for your convenience.
If you use this report to understand your revenue, those payments may be double-counted.
For example, when you sell a gift certificate via credit card you'll receive payment via credit card, and when the recipient redeems it you'll receive payment via gift certificate.
For this reason, you may not want to include gift certificate payments and voucher payments in your revenue. Or you may want to calculate revenue based on the Money Totals by Category report instead.
Your accountant or book keeper can advise you on this.