A specific price is calculated against the price of the product, meaning a discount on the original product price.
The rounding option is applied on the price of the product and will be applied on the discounted price thereafter.
To give you a concrete example:
A product with a price of 29.14$ with a $10 discount = reduced price of $19.14.
We then apply the rounding option X,X0 on the price including tax.
The price including tax becomes $29.00 and the reduced price becomes $19.00.
If the product includes a discount percentage (say 10%), rounding the price will change the discounted price by calculating the amount using the percentage:
Price including tax $29.14 - 10% = $26.23 reduced price
We apply the rounding option X,X0 on the price including tax $29.14: the product price including tax becomes $29.10 - 10% = $29.19 reduced price