Discounts are different from Price List Discounts (see how to create price lists for discounts and overrides) because with Price Lists you are adding specific or percentage discounts that can be unique to each product or service (if you so choose so). With discounts, this functionality allows you to apply a discount to any line item in an invoice as needed.
This video will walk you through how to setup different types of discounts in the VetLogix desktop admin app (app.vetlogix.io).