Discounts need to be created in Settings > Discounts before they can be manually assigned during a sale. You can create fixed and percent discounts. Both can be applied to individual sale items but only the percentage discounts can be applied to an entire sale. Optionally, you can select Require Customer to require that a customer be attached to the register at the time of the sale. This is helpful if you need to track which customers are given discounts in your reporting.
For more information, see our Creating discounts article.
AdvancedIf you need to automate your discounts, you can do so in Settings > Price Rules.
For more information, please see our Creating price rules article.