14 C.F.R. § 258.5 – Notice requirement.

(a) Notice in schedules. Carriers holding out or operating change-of-gauge services to, from, or within the United States shall ensure that in the written and electronic schedule information they provide to the public, to the Official Airline Guide and comparable publications, and to computer reservations systems, these services are shown as requiring a change of aircraft.
(b) Oral notice to prospective consumers. In any direct oral communication with a consumer in the United States concerning a change-of-gauge service, any carrier or ticket agent doing business in the United States shall tell the consumer before booking scheduled passenger air transportation to, from, or within the United States that the service requires a change of aircraft en route.
(c) Written notice. At the time of sale in the United States of transportation that includes a change-of-gauge service to, from, or within the United States, or, if no ticket is issued, no later than the time when the passenger checks in at the airport for the first flight in an itinerary that includes such a service, the selling carrier or ticket agent shall provide the following written notice:
Notice: Change of Aircraft Required For at least one of your flights, you must change aircraft en route even though your ticket may show only one flight number and have only one flight coupon for that flight. Further, in the case of some travel, one of your flights may not be identified at the airport by the number on your ticket, or it may be identified by other flight numbers in addition to the one on your ticket. At your request, the seller of this ticket will give you details of your change of aircraft, such as where it will occur and what aircraft types are involved.