This took a while to figure out as there are other forms of "security paper" in use. "inert" is also a quality similar to "dull" only that it did not react and reflect ultra-violet light which improved contrast for watermarks and other security codes that did rely on ultra-violet light to fluoresce and become visible. This is different from the "brightness" number (albedo) often printed on the wrapping of a ream of paper to indicate how much light it reflects.