Controls the entire cylindrical surface at once — roundness, straightness, and taper combined. Stricter than circularity, and harder to inspect.
In this Question Line video, Jason answers a question about reverse inspection of orientation and walks through an example to explain why you should not deviate from the inspection criteria given in the feature control frame.
Controls a surface or axis so it runs exactly parallel to a datum. Zero degrees is implied — no basic angle needed.
Controls a surface or axis at exactly 90° to a datum. Squareness has a formal definition in GD&T, and this is it.
Controls a surface or axis at an exact angle relative to a datum. Requires a basic angle dimension — it's the orientation call for everything that isn't 0° or 90°.