The title Grand Duke of Bosnia (veliki vojvoda) was a noble title of the Kingdom of Bosnia, bestowed by the King to the highest military commanders.
Unlike usage in Western Europe or Central Europe, as well as in various Slavic lands from Central to North-East Europe, where analogy between Grand Duke and Grand Prince was significant, with both titles corresponding to sovereign lower than King but higher than Duke, in Bosnia title Grand Duke corresponded more to Byzantine military title megas doux.
Like in rest of South Slavic neighboring states, and among their nobility, in Bosnia also existed the title knez or veliki knez, nominally analogous to Prince and Grand Prince, but in fact was medium to major feudal landlord, with corresponding influence in the Bosnian Stanak (also Great Bosnian Rusag, Whole of Bosnia), which was institute of assembly or council of all Bosnian nobility. Generally, Slavic word knez is often referred to ruler, sometimes analogous to King, thus veliki knez was more like High king than Grand duke.
In neighboring countries title Duke or Vojvoda also had military signification, however in that sense Grand Duke was specifically, even exclusively, Bosnian title.
Accordingly, title the Grand Duke of Bosnia was explicitly given by Bosnian ruler, at the time Ban or King/Queen respectively, to his/her highest-ranking military commanders. As such, it was an actually more like an office rather than a court rank, although it was also a grade in the court order of precedence, and was often held by one individual at the time, rarely two.