This may be over simplifying but here goes;
If a PC or Mon rolls a Natural 20, this by itself is now a critical hit.
If a PC or Mon rolls 10 above the target AC, this is a critical hit.
If a PC or Mon rolls a Natural 1, this is a critical fumble.
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.
You can check page 445 of the Core rulebook under Step 4: Determine the Degree of Success and Effect".
Essentially, statements 1 and 3 are false. Crits are determined by the result meeting or failing to meet a DC by 10 or more. If you roll a natural 1 or 20, the result is one degree worse or better than dictated by the numbers alone.
Hope that helps!