Enchanting “Shield” on clothing?

I’ve seen a couple places of people talking about doing it and I loved the idea… problem is, I’m not sure I’m doing it right?

I found a grand soul gem, filled it, put in some britches, it said “Self 9% shield” (or whatever) and I didn’t see a way to get it higher than 9%?

Am I right to think that “9% dmg shield” might be better than some higher defense number armors?

How do I make this make sense in my head… am I just trying to get 9% on as many things as possible to get a really high combined overall shield %? Does it even work like that?