You can set either percentage or specific value for a discount code. Set a limit on how many times the current discount can be redeemed on certain time period. If left empty there’s no upper limit.
A discount code created for a certain product applies to all it’s variants. If you wish to make discount for a specific product variant, a new product should be created.
You can fully edit your discounts. If you wish to edit the percentage, extend the campaign time or increase the redemption limit - you can freely do so at any time. Note that the changes will affect the new orders only.
Discount code will be counted as redeemed when a customer chooses a payment method in check out. This means that the redeemed code value might not always equal to payed orders, for example when the payment is cancelled.