A Battle Flag is an expendable item in Fire Emblem Heroes.
Battle Flags are exclusively used in Voting Gauntlets. These flags are used for fixed score multipliers upon winning a Voting Gauntlet Battle.
At the start of a round, players are gifted 10 Flags. Additional Flags can be obtained by clearing special quests during the Voting Gauntlet. Flags do not need to be used during the round in which they are obtained. Thus, Flags obtained in the Quarterfinals can be used in the Final round if collected and withheld from usage. However, all Battle Flags obtained during a Voting Gauntlet will be immediately lost at the end of the event, thus forcing players to expend them all to reap the benefits of their usage.