Viewability 100% is calculated as Viewable-CPM and you will only pay for banners that are defined according to the IAB standard:
minimum 50% of the banners pixels are shown for at least 1 second.
Formats
930×180
160×600
320×160 / 320×320
Formats
930×180
930×600
160×600 / 300×600
300×250
320×160 / 320×320