I know that this is a very simple question for some of you more experienced travelers.

I note that I can book different airlines on outbound and return flights? Does this cause any sorts of complications? I've always traveled on same carrier, for all parts of trip. On upcoming trip to RDU to Montana, I get some better options on Expedia and would like to try them.

Expedia tells me that seat selection is available on "some flights". What exactly does that mean?

Thanks so much.