I recently booked a flight for my wife and daughter on Aer Lingus (operated by Stobart). When I was booking, I noticed that in their drop-down menu for putting in one's Aer Lingus FF#, one of the other choices was United! I know that there are a few code shares between Aer Lingus and United, but specifically will there be any advantage to having a United FF# in this circumstance? Mileage credit? Upgrades? Does anyone know what the deal is?