Originally Posted by
mnbp
For a long connection, be sure to specify a KL flight number for the intra-europe flight, because even if a flight carries a DL code, DL sells its codeshares intra-europe only for short connections with DL TATL flights - and not for those long (yet completely within fare rules) connections.
However if wants to use GUC needs to use the DL codeshare number. Not that EU J is anything special.