Typically the step is a low flat bench, but my gym also has wooden boxes of various heights. I asked about this b/c I read about Russian and Bulgarian coaches who switched over their "clean and jerk" competitors from standard squats to step-ups. I don't know if they still do this, but the coaches talked about how it gave their legs a different sort of look to them and felt it added more mass. One of the guys was doing this exercise with like 315lbs or more.