X is not a graphical user interface. It is a network window system. The actual means of user interaction is left to the graphical user interface, which is based on the X Window System and managed by AIXwindows. You can run multiple processes. Each process has its own window.
All windows can be visible at the same time.
Size and placement of windows can be used to show importance. A window can run on a remote system. This means that the server controls the actual hardware used for input and output, and the clients make requests of the server. This allows client programs to be independent of the hardware. In X terminology, the display server is called either the server or the display.