Hidden Tax is the amount of VAT that a discount amount has. It is non-zero when all of these conditions are true:
When there is a discount that has tax embedded in it, Commerce computes a hidden tax that is added back in order to compute the discounted price.
discountedItemPrice = fullPriceWithoutTax - discountAmountOnFullPriceWithoutTax + vatAmountOnDiscountedPrice + hiddenTax
Full price of the item without taxes is: $100 / 1.2 = $83.33
VAT amount on the full item price is: $100 - $83.33 = $16.67
Can also be calculated as: $100 \ * (1 - 1/1.2).
Discount of 10% on $83.33 is: $8.33 (when we don't discount tax)
Discounted price of item with tax is: $100 - $8.33 = $91.67
This is the customer’s perception of how discounts are applied.
Discounted price of the item without taxes is: $91.67 / 1.2 = $76.39
VAT amount on the discounted price is: $91.67 - $76.39 = $15.28 (valid)
Can also be calculated as: $91.67 \ * (1 - 1/1.2).
Hidden tax or Discount Tax Compensation is the difference between the VAT amount of the full price versus discounted price: $16.67 - $15.28 = $1.39
Another way to look at this: hidden tax is the VAT amount carried within the $8.33 discount: $8.33 * (1 - 1/1.2).
How the customer usually understands the discounted price (Order Total):
Full price of item including taxes less the discount amount: $100 - $8.33 = $91.67
How Commerce calculates the discounted price (see earlier for formula):
$83.33 - $8.33 + 15.28 + 1.39 = $91.67*