When checking out an order, an operator can apply a discount manually. You can apply a discount on a Value basis and a Percentage basis. For this, go to the +Add Module and Click on Other Setting.

After that, you can see the boards of POS Discount by Value and POS Discount by Percentage. To add new discount values, Click on +button.

Now, a pop up will appear where you can fill the value and click on Save changes.