GD&T Key Terms -
Part 2

Actual local size is the value of any individual distance at any cross section of a FOS. The actual local size is checked at a point along the cross section of the part. A part FOS may have several different values of actual local size.

Unrelated Actual Mating Envelope is a similar perfect feature counterpart of the smallest size that can be contracted about an external feature or largest size that can be expanded within an internal feature and not constrained to any datum(s). 

Related Actual Mating Envelope is a similar perfect feature counterpart of the smallest size that can be contracted about an external feature or largest size that can be expanded within an internal feature while constrained to either in orientation or location or both to the applicable datum(s).

Actual Minimum Material Envelope is a similar perfect feature counterpart of the largest size that can be expanded within an external feature or smallest size that can be contracted about an internal feature so that it coincides with the surface at the lowest points. 
Unrelated Actual Minimum Material Envelope is a similar perfect feature counterpart of the largest size that can be expanded within an external feature or smallest size that can be contracted about an internal feature so that it coincides with the surface at the lowest points not constrained to any datum reference frame.