These settings are used in vectorization to specify vector line widths for contours based on their elevation interval.
For instance, you might set the elevation interval for major contours to 50, and the elevation interval for minor contours to 10. If you then followed a contour and set the elevation to 200, the resulting vector contour would be assigned to the major layer. Likewise, if you followed a contour and set the elevation value to 220, the resulting vector would be assigned to the minor layer.