Name Gallery - Slices (Image Slicing Properties)
Slicing is useful when exporting large graphics for use on web pages. Instead of exporting a single large graphic, you can export the graphic in sections or "slices". The viewer then sees the image build up as each slice downloads.
A second use is where a graphic has large areas of flat color. These areas can be exported as slices containing only a few colors (which makes the file size smaller). Slices containing detail can be exported with more colors.
Lastly navigation bars automatically export as slices.
When you choose Slices, a check box to the right of object names shows those named objects that slice underlying objects.