Top right side of pg 144 of Core Rulebook:
Extra damage over and above a weapon’s normal damage
is not multiplied when you score a critical hit.
What does this mean?
It primarily means extra damage dice, from sources such as sneak attack or the flaming weapon property, aren't multiplied when you score a critical hit.
Numerical bonus damage, such as from favored enemy or the weapon specialization feat, is multiplied.
Generally speaking rolled damage other than the base damage is not multiplied such as flaming weapon property, sneak attack damage, and so on.
Things like smite, the ranger bonus, magic enhancement bonus such as from a +1 weapon are all multiplied.
I have yet to find an exception to the above, but I can't account for new books or some random 3.5 ability I may have overlooked.
Top right side of pg 144 of Core Rulebook:
Extra damage over and above a weapon’s normal damage
is not multiplied when you score a critical hit.What does this mean?
It should say "extra damage dice."
So, for instance, let's consider a flaming two-handed sword +1 in the hands of a moderately strong person. It has a 19/x2 critical range. On a normal hit, it does:
2d6 (the two-handed sword's base damage) + 3 (+2 Strength * 1.5 for two hands) + 1d6 (Fire) (Flaming property)
=
2d6+3+1d6 (Fire)
On a critical hit, damage dice over and above the normal damage are not multiplied; they're just added once. So, with a x2 critical, this weapon and wielder will do:
(2d6+3+1d6(Fire))+(2d6+3) =
4d6+6+1d6 (Fire)
Make sense?
Yes, thank you. I would say that a Mace with 1d6 of Cold damage would be treated the same also.