To follow up, ASME Y14.5M - 1994, Page 139 (on my copy):
5.10 Noncircular Features
The fundamental principles of true position dimension and positional tolerancing for circular features, such as holes and bosses, apply also to non-circular features, such as open-end slots, tabs, and elongated holes. For such features of size, a positional tolerance is used to locate the center plane established by parallel surfaces of the feature. The tolerance value represents a distance between two parallel planes.
Perhaps if you included a scan of the drawing I could help further.