X clients programs should be written mindful of the fact that other clients are likely to be running at the same time. Windows should be prepared to be covered over by others then exposed, and they have to redraw themselves. Neither should clients indescriminately use scarce resources like off-screen memory or monopolise the keyboard or pointer. Any X application you write will need to work under a window manager. See the Motif manuals and on-line help for details.
X is extensible.
Neither the use or the writing of such extensions will be covered in this document.