In GD&T, symmetry tolerance establishes a tolerance zone for the median points of non-cylindrical part features. This tolerance is similar to concentricity, and the verification of symmetry tolerence is likewise time-consuming and difficult. It is generally recommended that position, parallelism, or straightness be used in lieu of symmetry tolerance.
An example of symmetry tolerance is shown below. The top figure shows the symmetry symbol applied to a notch. The boxed symbols can be read "the median points of these opposing surfaces must be symmetric about datum axis A within a tolerance zone of 0.7". The bottom figure shows and example part that meets this requirement. The median points must lie between two planes 0.7 apart and equally spaced about datum axis A.