"Destiny allows you to freely swap your ability choices, but according to Green there comes a point when you can lock them in. This gives your Guardian benefits, such as a stat boost. At this point you get a copy of the same focus, but that copy levels up much faster, thus allowing experimentation." http://www.eurogamer.net/articles/2014-04-30-how-destiny-turns-into-diablo-when-you-hit-the-level-cap The spot will be used for the new copy of the subclass after using illumination. In all honesty, adding a third subclass of the element the class is missing will remove any degree of meaning in choosing a class to play. As of right now, every class essentially has two strengths and a weakness when it comes to elemental damage, which makes choosing which element you want to have a weakness in have some meaning in character creation. If you add the third element to each of the classes, that disappears entirely, and completely gets rid of the purpose of ever leveling a second character.