I assume you mean in regards to a case study (probally Qantas?)
This may help:
http://en.wikipedia.org/wiki/Code_sharing
An example in Qantas is the code share aggreement they have with British airways. They can market and sell seats on each others flights
It enables the airlines to expand their networks while not having to spend money on infrasturcture such as planes and avoids the costs of setting up and running new routes etc. They are using the partners planes.