A position accuracy of High reports that you can fully trust the position, as the interaction source is within the sensors' visual field of view.
High is always reported for hands (see InteractionSourceKind and InteractionSourceHandedness on InteractionSource), as their positions are only ever reported during visual tracking. If you are making an experience that requires high position accuracy, such as a painting app, you may want to rely on positions reported with High accuracy and discard anything reported as Approximate. For simpler operatons, such as pointing at UI with the interaction source, Approxmiate is generally reliable enough.