Figuring out the terms used in the documentation

[Sleep]

 From the Xlib - C Language X Interface: "For each screen of the 
display, there may be a list of valid visual types supported at 
different depths of the screen."

Alright this is confusing me, this implies that one screen can handle 
windows with 8, 16, 24, 32 bits color depth at the same time and show 
the colors correctly. Is the color depth defined by the Visual or 
Screen? If Visual, does that mean that the same Screen can handle 
windows of different color depth or is that not possible because the 
root window being the parent of all windows on a Screen, enforces a 
certain color depth to be used?