Line guiding is one of the subsets of contrast sensing applications.
It requires the printing of a line on a web. It's purpose is to provide a feature that a sensor can track to determine the web position. The typical use is in web converting where guiding on the edge might not be a precise parameter in printing labels. However, printing a line is an additional cost. After all, it is additional ink. If the line is printed outside of the main printed features of the web, it could be leaving money in the converting floor in the form of excess trim on the edges of the web.