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