Ducat is the official currency of Principality of Gallia and is often abbreviated as DCT. Gameplay-wise, throughout the series, players can earn Ducat by completing missions. Achieving optional objectives such as killing enemy aces, destroying enemy vehicles, completing missions with minimal number of turns often rewards players with extra cash.
Ducat is usually used to research and upgrade weapons, armors and tanks. There are also a number of extra items that can be bought such as Irene Ellet's reports (VC1), newspapers and bonus missions (VC2), accessories (VC3).