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Almost nothing ...
Let's face it, guys.  Everyone who has spotlighted defense in their sabrmetric scope over the past decade HAS to understand one thing.  We STILL *know* a lot less than we don't.
I think everyone here understands that even using the best, most modern major metrics for defense remains essentially "suggestive". 
Honestly, I'd like to see home/away defensive splits.  Road defensive numbers, (IMO), would be VASTLY more useful for identifying INDIVIDUAL defensive skill, rather than organizational.  As a rule, I would expect the best defenders to be able to learn to play their own backyards superior to opponents.  If a team has a staff built to take advantage of a park, then there should be skewing.
But, road stats - where some of the parks will skew plus - some minus - for any particular staff, I think you're more likely to get a better read on the individual ability to defend, and suppress some of the organizational stuff.  I don't know anywhere to get Z rating splits for home/away -- but I'm hoping Matt can take a stab at doing that for PCA.