regarding domestic flights, each of the major network/legacy carriers have codeshare relationships with regional carriers in which they buy all of the capacity on a specific route, are responsible for selling it, and gain whatever profits are generated. In essence, the major carrier is simply putting its code on a smaller aircraft.
Foreign carriers such as DL's Skyteam joint venture partner KLM can codeshare and share revenue on either Delta mainline or Delta Connection flights.
Because Delta Connection carriers are legally separate, as are most regional carriers operating on behalf of their major airline partners, government processes have to be followed for each operating carrier. In Jim's example above, if DL operates one ATL-BHM flight and ExpressJet operates another and KLM wants its code on both flights, there are separate - but generally identical processes - that must be followed to obtain government approval.