Is the Actual Mating Envelope and Datum mean the same thing for this Feature of Size (10 mm Pin) ? No. they are different concepts. The datum is a theoretical surface (perfect points, lines, and planes). Measurements cannot be made from theoretical surfaces. It is the perfect cylinder of the pin, theoretical. The purpose of datumes is for the location and orientation of features, it is the origin from which the location or geometric characteristics of features of a part are established The mating envelope of the 10 mm pin, that happened to be also a datum, is not a theoretical surface. The diameter of this perfect cylinder can get bigger or smaller as required, and more ever is not the size of the pin but the size of the smallest hole in which the actual pin size goes in. So, it is an actual size, not a theoretical one. The actual mating envelope of a pin is the smallest similar hole counterpart that fits over the pin and contacts the pin at the highest points. There a