An order discount can be set via the Discount % field.
The % discount is applied to all item lines. Which can then be overridden individually.
You can create adhoc dollar value order discounts and surcharges in the POS area via the Surcharge / Discount Field
Applying a fixed dollar discount can use the same field as adding a surcharge. Just make it a negative amount, so that it comes off the order total.
Both discounting methods can be used simultaneously.