Discount refers to a reduction in the usual bill or invoice amount. In Web Suite, you can apply a discount to an invoice prior to processing it. You can do it in the Billing Review or Manual Invoice screen.
The discount reduces the final invoice amount after taxes. It does not reduce the tax amount of service or expense. However, you can change this setting in Global Settings-Billing panel. Proper security permission is required to apply a discount.
To apply a discount:
Web Suite reduces the Net Bill amount by whatever amount you enter here.