the vector ORIENTATION_D is coaxial to the cylindrical face of the feature and defines the Depth orientation.
the vector ORIENTATION_L is perpendicular to the _D vector.
For rotationally symmetric features like STEPPED features the ORIENTATION_L has no meaning.
For a features with WIDTH and LENGTH like POCKET_RECTANGULAR_STRAIGHT the ORIENTATION_L defines the direction of the LENGTH parameter.
The vector is normalized, if that is what you mean: the length = 1