The structural opening is the brick-to-brick dimensions of the aperture your door fits into. To measure, take 6 measurements (top, middle, bottom horizontally and left, middle, right vertically). If the aperture is slightly out of square, we would recommend using the smallest measurements to ensure square fit.
Your overall door frame size will be 6-10mm smaller than your structural opening size on width and height to allow for fitting. If you require a different tolerance, please contact the team to discuss.