Short form for understanding "essential": would someone be working at that position if there was a 4-6 foot snowfall and 35+ mph blowing winds outside? If you'd have to go in under those conditions, you're essential (at least for that day). If you'd stay home under those conditions, you're not. Biggest laugh of my life was seeing how many people considered themselves to be "essential" on the first day of furloughs under the Clinton-Gingrich showdown.