Imagine your OS story without one or the other. If problematic activities ceased, would the story be over? If not, then the OS Throughline is not Activity.
Picture an OS featuring both a war and a famine. Which is causing which? If the war is creating the food shortage, then the OS is Activity – if the fighting stopped, they could grow food again. But if the famine is causing the war, then cessation of hostilities doesn’t solve the problem – they’re still out of food. Thus an OS in Situation (Universe).
The source of conflict defines the domain.