What is the difference between using a nickname and an alternate name?

As far as Nickname field and Alternate name fact goes, they are for different purposes. The name in the nickname field can be called up in a sentence with either the :casual field option (which inserts the nickname, if there is one), or with the :first option (which uses the first name of the person (or persons for a family fact) if there is no nickname.) This allows greater flexibility and control when crafting sentences. On the other hand, the Alternate Name fact, although it has an option for Nickname, is a static fact sentence that cannot be called up in another fact. It does have a benefit of appearing in the Index panel at the lower left beside each view.