What are "selective" and "adaptive" color modes?

I'm using Adobe Imageready and noticed on the GIF-optimize tab that I have
four main choices in terms of the type of color output I want:  perceptual,
selective, adaptive and web colors.

Can someone explain to me what selective and adaptive are?  It appears it's
just a slightly different color palette (selective appears to be greatly
reduced).  I found on many images that I can choose "selective" and it will
cut my file size in half without degrading the image too much.  However, I'm
not sure why.  My guess is that it only uses the colors necessary to produce
the image, and discards any colors from the palette that aren't used.
Maybe?

Thanks,
Mike

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