Ability for the agency to provide (on the order form page) additional 'add-on' options for the customer. For example, if a client orders content writing, ability to offer a 'content optimization' service add-on for an additional fee. Another example would be upgrading a press release to include a specific premium news outlet.
How SPP does this is they let you choose add-ons from services you've already set up in the system. That way the pricing and everything is already set.