Once someone knows how to operate a piece of equipment or how to perform a particular process they do not need to refer to the step-by-step procedure. A properly trained operator needs only a few bits of information to do their job. These bits of information are what we term critical parameters.
It is desirable to keep critical parameters separate from your process documentation. Because these parameters change frequently, you want your operators to have easy access to them. If an operator must hunt through several pages of a process document to find the one bit of information they need, it is likely you will find operators using "cheat sheets" which may or may not be accurate.
Keeping critical parameters separate from the process procedures also helps keep your process documentation costs down. Developing and maintaining the process procedures generally carries the greatest cost. If you can reduce the need to make revisions this can reduce the documentation work load. Because critical parameters are usually quite volatile, keeping them out the process procedures reduces the need for revising the process procedure.
Critical parameters answer the question. "After the operator knows how to perform the process, what information do they need to complete a specific job?" Generally this information is contained on a sheet that comes with the job or is posted at the work site.