Marketing carrier is the airline that is shown on your ticket. It is also the one that appears on your itinerary. And with some kind of notation that this flight is operated by XX so you know to check in with the operating carrier. On most booking platforms the marketing carriers logo will be shown in request. And you are abiding by the marketing carrier’s fare rules.

as an example, I booked AY5777 CDG DFW. On the itinerary it showed AY7555 sold as AA49. In this case AY is the they carrier I booked on — so AY is the marketing carrier.