I had a UA code share flight on LH metal wanted an aisle seat reservation. Calling UA I was told that there is nothing they can do about. I had to call LH and got a different PNR and they did a seat reservation wich United had to confirm. I called three times LH and UA to make shure that all went fine and so I was told. Finally at the airport I ended up with no reserved seat They told me all happened because of the different PNR the airlines assign.