CreateNotify
The X11 event announcing that a new window has just been created as a child of a window you are watching — about the only thing a client can learn about a window before there is anything to show.
Nobody can have selected events on a window that does not exist yet, so CreateNotify goes to whoever selected SubstructureNotifyMask on the parent. It carries the parent, the new window's id, position, size, border width and the override-redirect flag. On the root window it means a new top-level window has appeared somewhere on the screen.
Window managers usually ignore it. A window that has been created but not mapped is invisible, and the event that matters is MapRequest, when the program asks to be shown. Compositors see it the other way round: they must track every window, including override-redirect menus that never pass through the window manager, so they start counting at CreateNotify. DestroyNotify closes the lifecycle.