What is the difference between a Code share agreement flight and a Marketing flight (business definition)?
A codeshare agreement, also known as codeshare, is an aviation business arrangement where two or more airlines share the same flight.
Sharing, in this sense, means that each airline publishes and markets the flight under its own airline designator and flight number as part of its published timetable or schedule.
And (emphasis mine):
A seat can be purchased on each airline's designator and flight number, but is operated by only one of these cooperating airlines, commonly called the operating carrier or more precisely (and in line with definitions in IATA Standard Schedules Information Manual): "Administrating carrier".
The term "code" refers to the identifier used in flight schedule, generally the two-character IATA airline designator code and flight number. Thus, XX123 (flight number 123 operated by the airline XX), might also be sold by airline YY as YY456 and by ZZ as ZZ9876. Airlines YY and ZZ are in this case called "Marketing airlines" (sometimes abbreviated MKT CXR for "marketing carrier").