Dave,
Here's my take on it: Let's say you have a process - as a simple example we'll say plating a piece of metal with copper. Let's say the spec. says the copper thickness is to be .00005" - .00009" thick, but you do not have the capability to measure this metalization thickness of the plated part. How do you ensure your process is good and puts out in-spec. parts? Let's say you precisely control the 3 process parameters X, Y, and Z that determine the metalization thickness within a narrow range and plating thickness measurements done by a special measurement lab show that so long as you control X, Y, and Z within those ranges, your plating thickness is always okay. As an auditor I want to see that you actually did the experiments (DOE would work well here) and had the measurements done, and that you follow the resulting procedure. Make sense?