This is used to show the actual print quality of fine lines.
One side is extruded out while the other side is extruded into the material.
Each number represents the height/depth of the lines and has a group of lines 0.2, 0.4, 0.6, 0.8, and 1mm in thickness.
Some lines do not print because the material doesn't allow it.
For example you cannot print a 1mm tall line unless it is 1mm thick.
Any line thinner than 1mm cannot print 1mm tall.