Essential definition substitution

Hi folks,

In anticipation of the agenda item tomorrow to continue the discussion of "essential" changes, I posted the following comment to the issue:
https://github.com/w3c/wcag21/issues/372#issuecomment-335249951

It's tough on the guy analyzing this that survey comments and call minutes continue to get wrapped up in only the 1st half of the "essential" glossary definition.  It's certainly understandable since that part is in line with how we might apply a dictionary definition.  But the problem is that when the 2nd half is considered, things stop making sense quick if not used correctly.

Please take a look at this substitution comment and either update the survey or discuss where my analysis has gone awry.  Hoping this will help smooth discussion tomorrow.  Thanks.

Steve Repsher
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