ASME Y14.5 : Dimensioning and Tolerancing

A similar perfect feature(s) counterpart of largest size that can be expanded within an external feature(s) or of smallest size that can be contracted about an internal feature(s) so that it coincides with the surface(s) at the lowest points. This envelope is on or within the material. There are two types of actual minimum material envelopes, as described below. (a) related actual minimum material envelope: a similar perfect feature(s) counterpart contracted about an internal feature(s) or expanded within an external feature(s) while constrained in orientation, in location, or in both orientation and location to the applicable datum(s). (b) unrelated actual minimum material envelope: a similar perfect feature(s) counterpart contracted about an internal feature(s) or expanded within an external feature(s), and not constrained to any datum reference frame.

See Also:

Feature, Datum, Datum Reference Frame