There are three possible scenarios involved in this question, depending on
the window types you have specified.
If you have rootless windows (in other words, each X-client application is
running in a separate Macintosh window), then each window's image is stored
locally in an offscreen bitmap. This prevents the necessity of having the
client application redraw the information.
The next two scenarios involve rooted windows, which are two different
issues.
The first issue is what happens to the entire rooted window when it is
forced to update itself. In this case, the currently-displayed rooted
window's image is stored in an offscreen bitmap, and the updates are done
locally and not by the client applications. This is consistent with the
method used by the rootless windows as mentioned above and is necessary,
because X client applications are typically not prepared to deal with
changes to the root window itself.
The second issue is involved with multiple X-client windows within the
single Macintosh root window. In the situation where you have overlapping
X windows within the rooted window, and you are switching between the
windows, the client applications are responsible for updating the clipped
regions just as they should be.
Perhaps the best way to think of this is that the images contained within
Macintosh-style windows are stored locally.