ASME Y14.5 : Dimensioning and Tolerancing

There are two types of irregular features of size, as follows: (a) a directly toleranced feature or collection of features that may contain or be contained by an unrelated AME that is a sphere, cylinder, or pair of parallel planes. (b) a directly toleranced feature or collection of features that may contain or be contained by an unrelated AME other than a sphere, cylinder, or pair of parallel planes.

See Also:

Feature of Size, Directly Toleranced Dimension, Actual Mating Envelope