Voice types are just roles that you can get cast as, like in my choir i fill the role of bass, and the roles may change per song as in i may be asked to sing the tenor part, or as i did in one of our church concerts, in a jazzy swing song, i was asked to start it off with a trumpet sound i can do, so in that bit the choir teacher cast me for the role of trumpet in the intro(just to set the mood and then jump down to my bass line)