There were two hetmans in Lithuania and two hetmans in crown of the Polish Kingdom: Field hetman and Grand Hetman, altogether 4. Grand Hetman usually was in command of the mobilised army (composed of gentry with their servants) and the Field hetman commanded the merceneries and artilery.
Both great and field hetmans were Senators of the Polish-Lithuanian Commonwealth
At the end of 16th century commander of the Cossacks were also called Hetmans. From 1648 Bohdan Khmelnycky uprising, Hetman was the head of the Cossack state. Cossack hetmans had very broad powers and acted as heads of the Cossack state, their supreme military commanders, the top legislators (by issuing administrative decrees). In the plans of reconstruction of the Polish-Lithuanian Commonwealth from dual into triple state (Polish-Lithuanian-Ruthenian Commonwealth), the hetman was to be a head of the Ruthenian part, consisting from 3 ukrainian Voivodships.