Detector coordinate (in pixels) along the direction of the fast changing array index (of the 2D diffraction data) for this reflection. This is often the position where the calculated reflection is predicted to occur. The fast changing array index of the 2D diffraction array is often also defined as the detector X-coordinate, while the slow changing array index is defined as the Y-coordinate. This can vary depending on the convention of the detector and the facility, especially for multi-panel detectors.