Starting from Winter 2015 Event, players can choose from the difficulty they want to play on each event map among three available difficulties: Easy (丙), Medium (乙), and Hard (甲). You need to have a HQ level of 35 for Medium and 80 for hard. Different difficulties yield different rewards with harder ones either giving more items or a better version of the same item. However, ship rewards are always given regardless of difficulty. Drops also differ with some ships available only on medium to hard maps, and drop rates for some ships are statistically better. Enemy compositions also differ with bosses and their escorts are usually weaker on easier difficulties. To finish the next map on Hard, you will need to finish the previous map on Medium or Hard.