Flatness – Surface vs. Feature of Size
Surface flatness is the type of flatness that most people are familiar with. On a drawing, the flatness callout can point straight to the surface with a leader arrow or be extended out from the surface and away from the feature of size.
Flatness - GD&T and Calypso Rev 3 Book
Flatness (form tolerance): the tolerance zone is limited by two parallel planes a distance t apart.
Implies: Straightness of the surface or Straightness of the derived median line