Block capped?

I've heard it said here or there and I have the excel sheet and don't fully understand how to use it.

What is the block capped situation and how do I know if i've reached it, also what is its importancE?
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80/Prot Pal/Azshara
http://www.wowarmory.com/character-sheet.xml?r=Azshara&n=Crawnick

Being block capped, as Honorshammer has written in his CAPtains guide, is the new term for uncrushable.

It just ensures that every normal hit that you'd take would become a block. It's a new face on the old 102.4%.

edit: Here's the link to the post:

http://www.failsafedesign.com/maintankadin/viewtopic.php?t=17075&highlight=
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Póg mo thóin