What?
Instead of just creating a broad-stroke interlining agreement with an airline, one has to define how the booking classes of one’s airline map to those of the partner’s. The agreement will also contain terms on how interlining segments are compensated (for example as a percentage of the default price, a fixed price, a prorated share of the total fare etc.) and possible where the agreement is applicable (although this might end up being part of fare rules).
Why?
With booking classes, this becomes necessary because the system can’t know which booking classes should be used when an interlining booking is made. But it also provides better insights into where traffic comes from, even without more elaborate statistics. This is because dedicated booking classes can be used for IL, meaning one can see exactly how much capacity has been used for IL. In combination with smart fare definitions, one could get a very clear, per-partner view of things.
When?
Depends on Custom Booking Classes and benefits greatly from Fares (base feature) and fare rules.