Form Is Not Fully Constrained
Not all aspects of form have been defined by the tolerances that have been applied to the feature or feature of size. The form of a feature or feature of size may be constrained in one of the following ways:
A form geometrical tolerances (flatness, straightness, roundness, or cylindricity) applied to the surface of the feature.
A non-form geometrical tolerance applied to the surface of the feature (e.g., perpendicularity applied to a planar surface imposes an equal implied flatness control to that surface).
A size dimension with the envelope requirement applied to a feature of size (i.e., form variation must be less than or equal to the size tolerance).
General geometrical tolerances, as specified by a reference to ISO 2768.
It is generally considered to be good practice to specify the limits of form for each surface of the model (either explicitly or implicitly).