Actually it's even more complicated than suggested previously. I posted a query in the newbie forum earlier in the week about MIA international transfers. I have done my own research since. It is not necessarily "airline specific" but can be "airport specific". Hold luggage transfer from international to international flights happens with ease at DFW but with more difficulty at MIA and JFK. If luggage is "Red Tagged" at original point of check in it is then IFI international connection luggage and is then transferred without collection at the carousel. Only hand luggage passes through US Customs in this circumstance. "Red Tagging" can be performed by any airline (if they want to) and is somewhat dependent on the airline. MIA website indicates that they allow this practice - what happens in reality is perhaps different! Best of luck with your travels!