Whizzwang is correct. However, a few armies, like Daemons of Chaos and Dwarves, can't take magic items at all. Your army book still has to say that you're allowed to take common magic items. You just aren't allowed to take the few common magic items they list, instead, you can any of the ones from the main book.
Just remember: only one of each type of magic item on each character, and only each magic item once in your army.
You also use the points value from the Army book, but the rules from the Rulebook for all the Common Magic Items. Bretoinian War Banner is 10 points Cheaper for example, as is the High Elf Dispel scroll.
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