Format object
A Format is one size a banner slot accepts, given either as an exact width and height or as a scalable ratio with a minimum width. Banners list them in Banner.format, and companion banners inside video and audio can carry them too. Added in OpenRTB 2.4.
How it is used
The ratio fields arrived in 2.5 for flex ads, whose size follows the container they render in. The spec recommends that each Format carry either the w and h pair or the wratio, hratio, and wmin set, not a mix of the two. The bid reports the size or ratio it chose in Bid.w and Bid.h or Bid.wratio and Bid.hratio.
Fields
Where it sits
Contained in: Banner.
All objects
The field reference index lists every OpenRTB 2.x object.