Here's the deal....
DME (Disney's Magical Express)..brings you to your resort from MCO and delivers your tagged bags to your room. Then, at the end of your stay, will take you (and bags if needed) back to MCO. This service is provided for every single person who arrives at MCO, on ANY airline, and who is listed on a reservation at a WDW owned reservation. You can use it if you are dropped off at MCO, if you arrive by plane, if you arrive the day before your reservation starts and you stay overnight at the airport.
RAC (Remote Airline Checkin).....allows you go check your bags at your resort and get your boarding passes issued if flying home on a 'participating airline'.
SW has recently begun participating at almost all resorts...(I believe CBR and OKW (I think) are the two that don't allow it yet due to storage issues). You do NOT need to be using DME back to MCO in order to use RAC. You can use DME, you can drive your rental car back to MCO and return it there, or you can use a car service...doesn't matter.