The arrangement is orderly, but the combination has no order. For example, Xiao Wang, Xiao Ming and Xiao Li choose two people as class committees. Choosing Xiao Wang and Xiao Ming is the same as choosing Xiao Ming and Xiao Wang because there is no order. This is a combination problem. And choose two people, one as the monitor and the other as the labor committee. Then, choosing Xiao Wang and Xiao Li is different from choosing Xiao Li and Xiao Wang. Because their positions are different, there is an order here, which is a matter of arrangement. Do you understand?