The thing I am not sure about: it kind of looks like weapons and armour have to have not just a potency rune of the desired value but the potency rune of every lesser value as well. So, for instance, a "fresh" +4 rune is the cost of a +4 rune plus a +3 rune plus a +2 rune plus a +1 rune. At least that is how I am reading it.
That's not how i read it. It's just that if you want to have lower potency runes on higher quality armour, they give you the math to do it.
Edit: I see what you're talking about now. YET ANOTHER SECTION. Yeah, that's Runes. They're to upgrade already existing magic items, so they only go on items that are already enchanted one lower.
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On the previous subject, though, regarding item bonuses... the Magic Armour does't say that the potency rune gives you an item bonus, it says it
increases the item bonus: "The armor’s item bonus to AC and TAC increases by the potency value"
...Though apparently quality armour only reduces its armour check penalty.
So... even though you have to look all over the place to find this... it appears that if the character that I'm working on has +1 Expert Scale Mail... then he gets:
(Pg176) Scale Mail gives AC +3; TAC +1; and -2 ACP
(Pg190) Expert gives a lowers ACP by 1
(Pg397) Magic Armour +1 says your item bonus increases by 1 to AC and TAC and you gain a +1 item bonus to Saving Throws
So... +4 AC, +2 TAC, +1 to Saving Throws and -1 ACP.
That took me forever to figure out.