Originally Posted by
TheoGeek
The Nat20 extension adds new "special" critical damage types - "nat20" (for a natural 20), "normalcrit" (for non-nat20 criticals), and "supercrit" (for crits that penetrate crit immune targets).
So, for example, a sword of sharpness only applies extra damage on a natural 20, not a mere critical. Some actors have a crit range of say 18-20, and without the Nat20 extension, a sword of sharpness would apply the extra 14 damage on an 18, 19, or 20 when it is only supposed to apply the 14 damage on a natural 20. So, adding an effect of "DMG: 14 nat20" causes the Nat20 extension to only include that damage if the attack roll was a natural 20.
I don't think there are any official 5E things that would use either "normalcrit" or "supercrit", but they are fun to play around with so I added them. :)