These two terms are actually a pair. The Shinogi is the line where the blade goes from the angular part to the flat part and the Shinogi-ji is the part between the edge and the flat part of the blade.
The higher the ridgeline, the more the Katana will be able to cut very hard targets, but will be slightly less effective on soft targets. This is why we make our Katanas with an intermediate ridge line, so that the blade is versatile enough.
The goal is to find the perfect compromise between strength, sharpness, and appearance so that the blade does not chip when cutting, but can also withstand many practice cuts.