Any player in any position can be made the captain.
As an assistant coach of girls volleyball teams, when looking for a captain I look for the player who has the following qualities:
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a captain goes at the beginning of the 1st game of the match to call the coin toss for their team. if they win, they decide if they want serve or side. if the match goes to 3 games the captain goes back for another coin toss.
They are the ones that can communicate with the ref. Before the game, they call heads or tails in the coin toss to see who serves first. In addition, they are the ones who motivate and lead the team.
The team captain in Volleyball is pretty much the person in charge on the court.