You “master” a tank when you earn more Base XP in a single battle than the highest average Base XP earned in a single battle by 99% of players of that same tank within the past seven days.
Simply put, just try and beat the average — doing so earns you 6x
Tank Mastery emblems!
If you attain mastery in one of the tanks below while the Tank Mastery mission is active, you also get a +100% (2 hours) 
Free XP Personal Reserve!
Restrictions: Grand and Random Battles only; once per account
