A sixth man is the first player off the bench, often a scorer good enough to start who gives the second unit a focal point.
In HoopSim the sixth man is whoever you put in slot 6 of the depth chart, the first bench slot, though bench order itself barely matters: the sim sends in the freshest, best available player who still has minutes and can play the open spot. What makes a real sixth man is minutes: give them a run in the twenties and they will get the bench's longest stretch early in the second quarter, usually alongside one starter who stays out as an anchor. Good players care about minutes, and a sixth man whose role shrinks a lot will say so, which shows up in morale. A sixth man also makes a natural closer when a starter is cold.
Rulebook: The 240-minute budget. Related: rotation, depth chart, closing lineup.