It depends on your settings. Two switches decide it: one on the automatic discount and one on the coupon.
On the automatic discount
When you create or edit a discount, open the Stacking Rules card and find Allow coupons to stack with this discount.
- Ticked: a customer can use a coupon on top of the sale.
- Not ticked: the customer gets the better of the two, not both.
Example: you put summer shoes on sale and want to protect your margin. Leave the box unticked, so a coupon does not add a second discount to those shoes.
On the coupon
The Use on Sale Items switch on the coupon decides whether the code works on items that are already on sale. Switch it off and the code only takes money off full price items in the basket.
Example: your code EID10 is meant for full price items only. Switch off Use on Sale Items on it, and items already on sale keep only their sale price.
On a Percentage coupon, Maximum Discount sets the most the code can take off.