Originally Posted by a380fan

Usually these are only available as part of an itinerary from select airports. Normally the airlines home airport(s) and perhaps other airports in their country or key markets. For BA it might be routes starting in London. Or itineraries starting in European cities with connections through London.

So imagining a hypothetical CX codeshare with BA from HKG to BKK. You couldn't just book this on the BA or CX website. But if you flew to BKK from LHR connecting in HKG on to a CX flight. Then that CX flight could be booked with a BA codeshare.

Codeshare rules vary though, some can be a lot more relaxed. Generally though the rules are similar to what I mention above.