In a feature that’s cylindrical or round, the circularity tolerance zone consists of inner and outer concentric circles that lie in a single plane. The zone defines the limits within which all the points of the circular feature must lie.
Circularity, also referred to as roundness, is a GD&T symbol that is frequently used in manufacturing features like a rotating shaft or bearing.
As a simple example, let’s say you have got ball bearings or spools that are used for moving (rotating) parts. Their roundness (circularity) has to be a tight enough tolerance to ensure these parts move smoothly and wear evenly. For the circularity of ball bearing, we have to consider it as applied for spheres. The circularity tolerance applies in all directions since any cross-section through the center of the sphere should be a circle.