Adding a Title Under Your Name

[Niwashi]

... and if that achievement gives a reward, it will usually have a specific icon beside it (a treasure chest for an item and a medal for a title).

There are three icons that can appear on achievement listings: a star, a medal, and a treasure chest. I know the medal icon indicates achievements that award titles and the treasure chest icon indicates achievements that award items, but does anyone know what the star icon indicates? It's the most common of those icons and appears on most (though not quite all) achievements, but I've never figured out what it signifies.