Chapter 11 explains these ratings, but I think they've been much overstressed.
You can have a cheap, poor-quality scope with an oversized objective lens and manipulate exit pupil and twilight factor ratings. You're stuck with third-class lenses, ground not nearly so well as another scope that didn't rate as high.
The Leupold Mark 4 is a good example. Its M3 military version has an exit pupil of four, which seems barely adequate, but the lenses are so well ground and coatings are of such high quality that it performs superbly in low light.
Exit pupils and twilight factors are only a guide and probably should not even be applied to cheap scopes.
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Deer hunting is an interesting thing that reminds you of those golden old ages of 19th centuries, where a handsome hunk well equipped with all hunting material rides on horse searching for his target animal either for the purpose of displaying his masculine powers or for enticing and wooing his lady love.