Roundness of a circle or sphere is evaluated by default based upon the Form Tolerance value provided for that circle or sphere. A minimum of four hits are necessary to determine the roundness of a circle, where a minimum of six hits are necessary to determine the roundness of a sphere. Additional hits provide a better representation of the roundness of the entire feature.
This dimension type is considered to be one sided, meaning the positive Form Tolerance value is applied. This means that any roundness that differs from your nominal or theoretical roundness can still be a valid measurement as long as it falls within the tolerance range specified.