Format
A mode of representation defined by an object's size, proportions, or viewport.
In practice
In this dictionary, format means the size and proportions of the designed object or its viewing area: an A4 page, a square social post, a tall phone screen, a wide cinema screen. Format sets the frame before any layout begins, and changing it often means rethinking a design rather than just resizing it. A festival campaign might need one message as a tall poster, a wide billboard and a square post.
Often confused with Export Settings
Format here is the size and proportion of the design itself; file type, such as JPEG or PDF, is chosen within export settings.
Try this
Adapt one simple poster to a square, a tall phone screen and a wide banner, noting what has to move.