I assume you're talking about visuals that look like this (from xdpyinfo):
visual id: 0x70
depth: 8 planes
available colormap entries: 256 per subfield
red, green, blue masks: 0x0, 0x0, 0x0
significant bits in color specification: 5 bits
This means that the visual has 8 planes, for a total of 256 colormap entries. Each colormap entry has a depth of 15 bits (5 bits per channel) for a total of 32,768 colors per colormap entry. The reason it's 5 bits per channel and not 8 is because the overlay hardware only supports depth 15.