Some measured features (circles, ellipses, lines, polygons, and slots) can be projected relative to a reference plane.
The Reference Type list controls whether or not the measured feature is 3D (not projected), projected relative to the current workplane, or projected relative to another specified plane. This affects mathematically how the final characteristics of the feature are derived.
For a detailed explanation on reference types, see the description of the Reference Feature area in the "Reference Feature" portion of "Using the Quick Start Dialog Box" topic in the "Using Other Windows, Editors, and Tools" chapter.