Many air travel involve various flight connections until reaching final destination. If you only have one reservation code and a single plane ticket number, it means you only have acquired one contract of transportation, even if you fly with two or more different air carriers. In situations of irregularities with some flights, for example, if the first flight suffers a delay, therefore, makes you miss the flight connection, the first flight company takes responsibility for the assistance providing and for the re-routing to the final destination.
However, in situations where you have more than one reservation and plane tickets with different numbers, each ticket constitutes a separate contract of transportation. In case of irregularity with some of the contracts, you may not be covered and have additional costs purchasing a new plane ticket to reach your final destination, for each air carrier is responsible for its contract of transportation.
Separated contracts of transportation may be cheaper, in these cases, it is advised to give space between the flights (preferably a 24-hour gap) so that in the case of irregularity with some of the flights, the connections or any of the following transport contracts will not be lost.