A referendum shall be held:

(a) Whenever prescribed by the order;

(b) For the purpose of adjusting the rate of assessment:

(1) At the direction of the Secretary; or

(2) Upon request of the Board or upon request of any group of fluid milk processors that marketed during a representative period, as determined by the Secretary, 10 percent or more of the volume of fluid milk products marketed by all processors of fluid milk in the United States during that period; or

(c) For the purpose of suspending or terminating the order:

(1) At the direction of the Secretary; or

(2) Upon request of the Board or upon request of any group of fluid milk processors that marketed during a representative period, as determined by the Secretary, 10 percent or more of the volume of fluid milk products marketed by fluid milk processors voting in the preceding referendum.