In your case, I have no idea.
I do know that I recently bought a ticket on NW from SFO to Osaka. Since they only serve Osaka from Detroit (I think), it was a legal routing to fly NW to NRT and connect to JAL or ANA for a domestic flight into either KIX or ITM. It's somehow all covered by the NW SFO-OSA fare, though, even though the domestic flight isn't a NW codeshare.