Re: Second Filter
With herbology, we would need someone who can take care of the ingredients to Snape's exacting standards. I'm not sure the house-elves can. Cassandra was doing it, but- well, we need someone else.
Now, about the events, would you like to be in charge of birthday parties? Those are completely not part of the official job, but basically it's just reminding people when the party is and telling the house-elves to do cakes.