On your Media Campaign you have the option to set three discounts:

1. Quantity Discount
2. Customer Discount
3. Agency Discount

All of these discounts are used to calculate the Net 1 from your Amount and get inherited to all campaign line items. Remember, there is no special discount on the media campaign, only on line items. The special discount absolute field is a roll-up calculation suming up the special discounts on your line items. To understand the calculation scheme and hierarchy of net and gross amounts, take a look at the 4.2.6.2 Amount Calculation and Discounts page.