Superogatories are actions that are commendable, but are not required. Duties are required and assigned actions.
Superogatories as defined as "actions that are commendable, but not required." Examples of such actions are when a person risks their safety to save someone else's life, like someone who is drowning. Very commendable but not his or her duty to save the drowning person. There is no moral condemnation of the people who passed up the drowning person because of the weight of the value of their life and safety in their decision.