If you have collected a whole Set of Gear, Showroom will enable their Set Boost; if you own multiple Sets, those Set Boosts can also be activated at the same time. Even if your Hero doesn't equip with these Gear, their Set Boost can still be on. The level of Set Boost are decided by the lowest-quality Gear in a complete Set, but also limited by the level of Showroom. So if you want to activate a higher level boost, you need to upgrade your Showroom. Unlock the Showroom, and unleash the true power of your Sets!