The total price reduction through discount & voucher codes and cart rules.
Formula
Discount Code Rate = Discount Code Value / (Gross Merchandise Value - Price Reductions)
Explanation
The Discount Code Value shows you by how much the Checkout Price of your orders was reduced because people applied a voucher/discount code. Discounts on Shipping Charges are also included in this number.
Notes
We differentiate between Price Reductions & Discount Codes.
The Voucher Value contains reductions to specific line-item with a product's share of any cart-wide discount.
Building up on the example for the Price Reduction. When you are selling a product that originally costs €50, but you then reduced the Price in Shopify by €10 to €40 on which the customer uses a 10% discount code, then:
Price Reduction: €10
Discount Code Value: €4
The Discount Code Value also includes discounts granted on Shipping Charges.