This may be over simplifying but here goes;
Statement 1.
If a PC or Mon rolls a Natural 20, this by itself is now a critical hit.
Statement 2.
If a PC or Mon rolls 10 above the target AC, this is a critical hit.
Statement 3.
If a PC or Mon rolls a Natural 1, this is a critical fumble.
Statement 4.
If a PC or Mon rolls 10 below target AC, this is a critical fumble.
Presuming all the above Statements are correct what happens when a Natural roll(either 1 or 20) is also 10 away from target?
I'm just using combat as an example , but this could also apply to other types of Successes.