Circularity is used to control how circular a feature, like a hole or a cylindrical surface, must be. It limits deviations from a perfect circle. Circularity is inspected only as individual circle elements within a single plane at a time. It does not limit the location or orientation of the circular element relative to any other feature.
Circularity is a refinement of size. For example, a circular feature might have a large diameter tolerance, but may need to be quite round.
In the example below, notice that circularity only “sees” the elliptical section at the end as being an imperfect circle. Circularity is does not “see” taper, eccentricity, or errors of straightness, location or orientation.
The part below is tapered, but only fails roundness in the area where the notch is present.
The part below is round but not straight, and it is not cylindrical.
