When we look at the modeling scheme, we see four nodes representing 'states of affairs', or 'states of the world'. For example, if the state of affairs is such that there is an apple on the table in front of me, a can reach it and pick it up. If there is no apple this is impossible. If the moon is on the night sky, I can execute the action of observing it. For this purpose I rotate my head in a certain way and keep my eyes open.
Observation is a kind of action.
But there are states of another type, which do not fit this definition. If I feel pain, or am frustrated, or elated, angry, or complacent, this has no effect on the actions I can take. It affects only the choices I am going to make selecting from the same set of possible actions. But, obviously, the more it hurts, the more likley I am to withraw it. After all, we register an action when the states of the world changes.