18 CFR §35.8

Unless the notice issued by the Commission provides otherwise, any protest or intervention to a rate filing made pursuant to this part must be filed in accordance with §§ 385.211 and 385.214 of this chapter, on or before 21 days after the subject rate filing. A protest must state the basis for the objection. A protest will be considered by the Commission in determining the appropriate action to be taken, but will not serve to make the protestant a party to the proceeding. A person wishing to become a party to the proceeding must file a motion to intervene.