Often two holes are used as datum features. There are several ways to accomplish this. The two holes may be used as a pattern (linear extruded feature) or one may serve as a datum feature that establishes an axis (origin of measurement) and the other orients the two perpendicular datum planes that intersect at this axis and, of course, the modifiers applied to these datum features will have an effect.
Keep in mind that parts are made up of features and datum reference frames (composed of 3 mutually perpendicular datum planes) exist in the processing equipment. Processing equipment should always include the inspection equipment and sometimes the production equipment. Datum feature referencing in feature control frames indicates how Inspection is to align a part to the datum reference frame. Often Production may take advantage of this to assure that they are producing “in-spec” parts.
On this part datum feature A establishes the first datum plane of the datum reference frame. The hole labeled datum feature B establishes the origin of measurement based on the related actual mating envelope (biggest cylinder, like a gage pin, held at 90° to the first datum plane or electronically it would be the largest inscribed cylinder that is perpendicular to the first datum plane) that fits the hole. The actual mating envelope of datum feature C, which is related to the first datum plane and the datum axis established by datum feature B, is used to orient the planes of the datum reference framework. Any tolerances that reference A, B, C will use these datum planes to establish an origin of measurement.