Local vs Global modifiers

A local mod affects that property of the item, as opposed to a global one which affects it for your character. Thus a local mod can only occur on an item with that property, and conversely, global mods will not occur on such items.

So, purely for example, "% increased physical damage" on a ring, can't be local, because rings don't have base physical damage. "% increased physical damage" on a weapon is local, and affects the value of the weapon, rather than you. You'll see that the damage values on such a weapon are blue, rather than white, to show they've been modified by local mods.

The global version of a mod will (almost) never spawn as a random on something that it could be local on. You can get global crit mods on weapons, where they could have been local, but only as implicit mods (daggers give global crit chance, and thrusting swords give global crit multiplier). In those cases, to prevent any confusion, the global mod is specifically labelled "Global" so you can see it isn't local.

So, if you see an "% increased armour" mod on a helm, that's local, not global, and the armour value of the helm will be in blue to show that that's a modified value. If you see "% increased armour" on a ring, that's global.