Having set up a partnership with another airline you can offer codeshare flights to your partner (and vice versa). Codeshare flights are routes offered by one airline to another which can be flown by pilots from both airlines. The airline offering the codeshare doesn't have to change anything and when the codeshare is accepted a 10% bonus on passengers is given on that flight. The airline accepting the codeshare can fly that route, too (with their own aircraft and pilots) and will also get the 10% bonus on passengers.
So let's say you're a longhaul carrier and your pilots would like to do some short regional flights, too. Instead of adding new routes and breaking or airline policy you can look for a partner airline which offers you regional codeshare flights. If you always have full airplanes on your flights you can also increase the ticket price on codeshare flights and still get the same number of passengers.
Go to the partnerships page, click on the info-button next to Codeshare routes and then choose Offer a new route
Go to the partnerships page, click on the info-button next to Codeshare routes and then click on the green tick next to the red route the airline is offering you. After that you have to assign an aircraft or aircraft type to that flight. Having accepted the codeshare route, the flight will show up in your 'flightplan' and can be booked by your pilots.