What counts as a "visual stimulus" is unclear to me. Does it mean a static object, as opposed to a blank surface or something that moves, and if it is something

What counts as a "visual stimulus" is unclear to me. Does it mean a static object, as opposed to a blank surface or something that moves, and if it is something that moves, does it move predictably (ex: a rotating fan) or not (ex: a notification). It is kind of obvious that stimuli like notifications are disruptive, but less so in the other cases.