In a perfect world, all characteristics could be parameterized. Unfortunately, the world is less than perfect: some requirements remain “qualitative” and, therefore, unmeasured. That situation is often problematic, because it is difficult to determine whether or not such requirements have been met.
In many cases, non-quantification can be addressed through the use of a surrogate verification requirement. That approach will, of course, require prior concurrence from the Acquisition Customer (and possibly the End User, too).