Remarks

A Helmert datum shift can be used in both directions, but it will have one natural forward direction (where the Helmert formula is used directly), and one backward direction (where the rotation matrix must be inverted before use). The Direction property tells whether the forward direction goes from the owner datum to the known datum, or vice versa. If the Helmert datum shift comes from the EPSG database, the direction will be to the known datum, which is usually WGS84.