Elemental Mastery is the stat that affects Elemental Reactions the most. It adds a percentage bonus to Vaporize, Melt, Overloaded, Electro-Charged, Shattered, and Swirl reactions. Vaporize and Melt have a different rate than the rest. It also increases the damage absorption of shields created by Crystallize reactions. If you're going to maximize this stat, stack it on the character that will initiate the reaction. For example, Kokomi is a suitable Hydro applicator to set up Vaporize reactions with Klee. Unless the enemies tripped on Klee's bombs before Kokomi released her jellyfish, Klee would create the Vaporize reaction. Anemo characters primarily have skills that scale off EM. Sayu has a passive that increases her healing proportional to her EM. Her C6 upgrade allows the stat to affect her Burst too. Also, they can do the Swirl reaction, which can also cause other combinations by spreading elemental statuses. That means they can potentially create a cascade of reactions, all of which benefit from this stat.