In addition to what others said, it also may represent the income from royal domain - in many countries monarchs owned considerable amount of land and in many cases national treasury and monarch's own money was basically the same thing. In Poland, for example, in that period main sources of money for the state were not really taxes (as nobility and clergy had many privileges and exemptions from taxation and collecting taxes was problematic) but mostly state monopoly for salt mines (hence in vanilla Eu4 Kraków province is very rich and has salt as a trade good), income from vast arable lands and cities owned by the king and custom taxes imposed on trade. Also, for example, in Russia, Peter the Great established state monopoly on alcohol production, so all the money from alcohol consumption in Russia went straight to the treasury (and btw, he also implemented laws encouraging alcohol consumption among peasants, which some claim led to alcoholism problems in Russia). so, basically, the state had quite big impact on economical life back then.