14 CFR §221.53

1. (a)Definite application. Add-on fares shall be specifically designated as “add-on” fares on each page where they appear.
2. (b)A tariff may provide that fares from (or to) particular points shall be determined by the addition of add-ons to, or the deduction of add-ons from, fares therein which apply from (or to) a base point. Provisions for the addition or deduction of such add-ons shall be shown either directly in connection with the fare applying to or from the base point or in a separate provision which shall specifically name the base point. The tariff shall clearly and definitely state the manner in which such add-ons shall be applied.
3. (c)Restrictions upon beyond points or connecting carriers. If an add-on fare is intended for use only on traffic originating at and/or destined to particular beyond points or is to apply only in connection with particular connecting carriers, such application shall be clearly and explicitly stated directly in connection with such add-on fare.