[Calahan]

Resistances from different sources mostly always stack, although there are some things to bear in mind when you consider the effects from spells and items.

For example, a mage with a Ring of Frost won't cast Cold Resistance (CR hereafter) on himself, as he is already getting that effect from the Ring of Frost. And it doesn't matter if the Ring of Frost is giving him 100% cold immunity or not (as other items, like Marble Armour, may be bringing the CR down). But he will cast Breath of Winter to boost his CR regardless of what his current CR is. Although of course he won't cast it if he is wearing a Rime Hauberk, as that grants the wearer the Breath of Winter effect.

So the key thing to remember when it comes to item/spell stacking for resistances, is that the exact same effect from two or more different sources do not stack. Above is just two examples, but there are several more.