In the past, codeshares often involved blocks of seats primarily due to reservations systems limitations. The CRS's could not communicate with each other in "near" real time. Therefore, in order to avoid overselling a codeshare flight, the airlines divided up the seats into blocks.

Today, for the most part, the CRS's can communicate in "near" real-time and codeshares no longer involve specific blocks of seats. Rather, the two airlines just book seats until the flight is full. Of course, the operating carrier always reserves the right to deny any more codeshare bookings -- even when the flight is not yet full. For example, this might happen when the codeshare partner is offering lower fares than the operating carrier.

Of course, there are exception to this rule. Some CRS's are not yet well connected (particularly true of smaller International carriers). Also, the airlines may restrict premium cabins to "blocks of seats" to have more control over the "expensive" seats.