Normally advert placement areas are populated by adverts where each advert is specifically assigned to that placement arera when it is created. You may elect to fill up a placement area with other adverts by checking off the "types" of adverts that can fill that placement.

Conversely, you can also not assign individual adverts to a placement, and just use the fill types check boxes to specify all advert types that can fill the placement.

In the end, it all depends on the type of control you want to exert over which adverts appear in which placements.