Putting it very bluntly;
Originally Posted by Olias
The block% you see in the Defenses tab is the chance you'll block against a mob the same level as you are.
Scrolling over it, the "Block Rating of 0 adds 0.00%" means that you have no gear that has the "Block Rating" stat on it, which purely increases your chance to block. It's exactly the same if you were to scroll over the Dodge or Parry stats from the Defenses tab, except unlike those, Block rating does not have a Diminishing Return on it.
The block value is obviously how much damage you do block.
As a quick example from a warrior in my guild;
He has 572 Defense (400 base, 172 from gear), 2 points in Shield Spec, and no "Block Rating" on his gear. This means he has 5% (Base 400 defense) + 2% (shield spec) + 6.88% (0.04% for the 172 Defense from gear) for a total of 13.88% block. His scroll over would show 0 Block rating for 0.00% then his BV number.
If he puts on his block value set, he has 584 Defense, 2 points in shield spec, but a lot of gear with Block rating on it. He then has the 5% + 2% + 7.36% for 14.36% from those three previous sources, but he has 325 Block Rating which adds 19.82%. This means in the Defenses tab he'll see the 34.18% block, but his scroll over will show "Block Rating of 325 adds 19.82%."
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