The term is somewhat misleading. 'Essential' is limited to the people without whom something would fail in a rapid and spectacular way, or the people who would actually have to sign something in an emergency or take a flash call, or people required by some law or regulation to be there (Like the guy whose name is on the blame line for a nuke license, or a certified operator for a water purification plant). It doesn't mean the place could actually operate and do whatever it is supposed to do beyond just answering the phone and taking notes without all the 'Nonessential' people (Many of them, yes, all of them, no way). Being designated 'Essential' also means, for instance, that you can't be a member of the Reserves.