When making a sale there could be an additional discount apply to the whole sale. You may do so like this
Main menu > Settings
Sales Orders
Orders > Allow Additional discount > ON
Now on the order from
Main menu > Database > Sales / Invoices > All orders > select the order to apply the discount
Touch the yellow label Additional Disc.
Type in the amount and select the type like money or precentage then save
Also you may have a discount on any item of the basket. Touch the yellow price tag to select Discount %
However you have also another option when creating the basket. On the pre-basket window touch the box-penciled
Scroll the pop-up on Additional Discount select the option and type the value of the discount to apply to the whole sale. then touch Finish order or Finish order and invoice