Stacking items and runes

I dont really get it, because im looking at some third eyes for my ele, and some of them you can stack, and tohers you can. And with runes some you can stack and some you cant. Please somene explain..

You can only wear one head piece at a time and the +1 from it will stack with a rune.

Runes do not stack. Two +1 runes = +1, a +1 and +2 = +2, etc.

If you equip 2 of the same major or superior you will suffer the hit point loss even though you don't gain the attribute bonus.

Oh. I see, thanks