I know on some codeshares I have flown, I've been told by airline employees there has been a specific "block" of seats that are assigned to the codesharing airline's passengers. This seems to also occur when a cruise ship buys a block of seats fore resale.

Fortunately, I have been able to effectuate seat changes once I have arrived at the airport (early, of course,) with the hosting airline's personnel - often to exit row seats, which are generally held until the day of flight by most airlines - as the "block" never seems to have the most desirable seats in it - the seats are in the aft half of coach, for example

Perhaps others who are more directly knowledgeable in airline codesharing ops will post additional and more accurate information?

Ah, hah - see formeraa's explanation below!