Answer: Pt. (Caliper)!
When it comes to papermaking, three variables come into play: weight, caliper (or thickness), and smoothness. Think of it like a 3-ring circus, and the mills can only control two out of those three rings.
Paper is made by using a specific amount of fiber and fillers to create a sheet of a fixed weight. To achieve a certain level of smoothness, the fibers get highly compressed and smoothed by calendering (think: ironing). This process causes the paper to lose thickness and therefore stiffness, but the weight of the fiber is still the same.
Paperboard, C1S, C2S and reply cards are all called out in points (Pt.), meaning the caliper and stiffness of the sheet are guaranteed, which is essential when it comes to packaging projects.
PRO CALIPER GUIDE