Screen Values
Highlights - Shadows - Light Screens
The standard rule for printing images is to be sure all highlight detail has density of at least 5%, and all shadow detail is no darker than 95%. So apart from specular pure white or pure black, the image density should lie between 5% and 95%.
These densities other than pure black and pure white are also called screen values because they are produced in printing by use of a dot pattern with varying size dots, and this pattern is referred to as a screen.
Along the same lines, any light screen value that you want to print reliably should have density of at least 5%.
Manufacturing Tolerances - For guidance on dimensional manufacturing tolerances, visit here