The scale difference increased by unity equals the ratio of an the length of an arbitrary distance between two points in target and source coordinate reference systems.  This is usually averaged for the intersection area of the two coordinate reference systems. ¶¶If a distance of 100 km in the source coordinate reference system translates into a distance of 100.001 km in the target coordinate reference system, the scale difference is 1 ppm (the ratio being 1.000001).