What is regular price and sale price ?
Regular price is the normal price set for the product.
Sometimes, the owners would like to provide a discount directly. So a sale price will be set.

Now, you can choose from which price the discounts should be calculated.
Yes, you can choose from the settings tab :
In this way, the discounts will work based on the selected option:
Navigate to Woocommerce --> Woo discount rules --> Settings tab.
You can see the option Calculate discount based on.
Here, you can choose between sale price and regular price from the dropdown.

Note :
Only either of one price can be chosen, (i.e) You can select either sale price or regular price(not both).
If sale price is not set for a product, then the regular price will be taken into consideration.
If you are trying to apply discounts when using plugins like add ons and bundle, make sure to apply discount from the sale price.
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