WM_HINTS
The ICCCM property carrying an X client's small requests to the window manager: whether it wants keyboard focus, whether it should start as an icon, what its icon is, which group it belongs to and, since 1994, whether it wants attention.
The property is set on the top-level window with XSetWMHints and holds an XWMHints structure whose flags say which fields are filled in. input tells the window manager whether the client wants to be given focus or will handle the matter itself. initial_state is NormalState or IconicState, so a program can be launched already minimised. icon_pixmap, icon_mask and icon_window supply an image or an entire window to show while iconified, and window_group names a leader window so that all of an application's windows can be iconified together. The dockapps of Window Maker and AfterStep are a trick built on exactly these fields: start in WithdrawnState, point to an icon window, and the dock displays it.
ICCCM 2.0 added XUrgencyHint, the urgency flag. A client that sets it asks for the user's attention without stealing focus, and that is where blinking taskbar entries and orange title bars come from; EWMH later gave the same idea a second name, _NET_WM_STATE_DEMANDS_ATTENTION. The name invites confusion with two relatives: WM_NORMAL_HINTS is about size, and _MOTIF_WM_HINTS about decorations. xprop prints the hints in plain language, starting with the line "Client accepts input or input focus".