When using the Precision Mic to measure the case head-to-shoulder dimension the 'zero' mark on the thimble corresponds to SAAMI minimum chamber headspace dimension, e.g. 1.7867" for 6mm Remmington, 1.630" for .308 Win etc.
Although the tool is often used for relative measurements (e.g. setting the shoulder back 0.002") it is also useful for making absolute measurements, e.g. confirming that a case conforms to SAAMI dimensions or adjusting dies to achieve the same. The table in the instructions lists the chamber min/max dimensions for various calibers.