The mastery sets in indie superhero MMORPG City of Titans are a bit like specializations for the game’s archetype. In addition to each archetype’s power sets, your mastery set lets you do some extra things to focus in your playstyle from a few different options. The latest preview on the official site shows off one of the mastery sets for each archetype, demonstrating how each one allows you to tilt the same powers in a different direction.
For example, the Surprise Strike set for Enforcers allows you to build your archetype more toward the stealthy side of things, giving extra bonuses against unaware targets. Saving Grace makes your guardian more dangerous to fresh enemies and better able to sustain your group through heavy fire. Focus makes your Operator better able to lock down the field for a group. It’s not enough to make for a totally different experience, but it does allow you to tilt your play a bit into a specific direction, which is ultimately a good thing.