Armor bonus questions, please help!

Does a bonus on one piece of armor give a global effect to the whole set that you wear, or just that one area?
I've noticed a bunch of warriors wearing mostly gladiators and an ascalon helm.
Does the total armor level benefit from having that one piece?

The reason I'm asking this is so that I can gain a much deeper understanding of how the armor works in this game before I make a major decision on which armor to craft.

If you mean like the dmg absorption on Knight's armor, no they don't stack. It's specific to each individual piece of armor. Also, there is no 15k glad helm and a lot of people grab the ascalon helm just cause they like the way it looks together.

it used to work, but style hasnt changed yet

It kinda works on a case by case basis. Used to be that 1 Knight's piece led to global reduction, but that's no longer the case.

The reason for the Ascalon helm is the +1 to tactics, I believe, for instance.