I believe it to be a digital adjustment for the amount/type of light it lets in. If you white balance for indoors, it helps reduce the yellow given off by many lights, and makes it a more natural "light" color.
If calibrated for outdoor white balance, it will likely do the same for the sun, but reduce the brightness a bit.
Haven't played with it much, as I'm still relatively new to DSLR's. It's not a function that was on any of my old 35mm cameras. But this is how I've come to understand it.
Rick
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