It's covered under the Airline Access Act.
§ 382.37 Seat assignments.
(c) If a service animal cannot be accommodated at the seat location of the qualified individual with a disability whom the animal is accompanying (see § 382.55(a)(2)), the carrier shall offer the passenger the opportunity to move with the animal to a seat location, if present on the aircraft, where the animal can be accommodated, as an alternative to requiring that the animal travel with checked baggage.
§ 382.38 Seating accommodations
(a) On request of an individual who self-identifies to a carrier as having a disability specified in this paragraph, the carrier shall provide the following seating accommodations, subject to the provisions of this section:
......
(3) For an individual traveling with a service animal, the carrier shall provide, as the individual requests, either a bulkhead seat or a seat other than a bulkhead seat.
There's some more stuff about giving notice and emergency rows, but for the most part service animals in cabin are federal rules not airline policy. The basic outline seems to be if you give them notice the airline is required to move people to accommodation you.
I've seen from time to time questions about getting split up from elderly people or children. The act also requires the airline to keep declared caregivers seated next to people declaring ADA needs.