The assignment of a particular grade to a quality category and the establishment of sales policies are made by each paper mill based on its own internal evaluation of its products relative to those of its competitors. That makes direct, objective, competitive comparisons between different papers virtually impossible.
The color of paper is identified using adjectives like "cream","natural", "white", etc. rather than objective definitions such as the CIEL*a*b* coordinates that printers use. Again, there are no specifications or tolerances provided by mills as to the consistency of a specific paper color either through the stack, roll, or when the same brand is supplied by mills located in different regions.
The brightness of the sheet is one common measure of distinction. It is measured by comparing the amount of light reflected by the paper surface to the amount of the same light reflected by the surface of magnesium oxide established as the standard of 100%. A common term used in quoting the measure is “G.E. brightness,” although G.E. no longer manufactures the measuring instrument. "Brightness" is also not a measurement that printers or buyers can measure themselves. Even though variation is part of every manufacturing process, there are no defined tolerances for paper brightness.
There are no specifications or tolerances or even notification of optical brightener agent content. This has become a major issue as the mills have mostly switched from clay fillers to calcium carbonate. This problem has resulted in the fact that papers today, for the most part, no longer meet the ISO 12647 specification. Papers with optical brighteners are impossible to visually match between printing technologies which can cause severe disconnects between proof and presswork as well as greater color shifting as presswork and proof are viewed under different lighting conditions.
"Caliper" defines the thickness of paper, measured in thousandths of an inch, which can also be expressed as a point size - e.g. If the caliper of a paper measures .009 inches it is a "9 point" stock. As paper caliper varies, presswork color may also vary. But again there are no defined tolerances for paper caliper either through the stack, roll, or across the width or length of the sheet.