What I think this means is that out of the 256 possible colors, these are the ones for which we have the greatest ability to discern the differences. Selective is the Photoshop and ImageReady default option for Color Tables and produces images with the greatest color integrity. Similar to Perceptual, Selective concentrates on broad areas of color and attempts to use Web Safe colors as much as possible. Adaptive builds the best palette of colors based upon the selected image. An image with a predominance of red for example, will have more variations of red than any of the other Color Tables.
Web is the Web Safe palette.
All colors are composed of one or more of the 216 Web Safe colors. While the image quality is not as good as the other tables when used with photographic images or images with subtle gradients, the resulting image will appear about the same on all computer platforms. Custom preserves the settings for the selected Color Table and does not change when the image is modified. Mac OS uses up to 256 colors that are the default system colors on the Macintosh platform. 216 of these colors are Web Safe, the remaining 40 are not.