TechCrunch’s Matthew Panzarino has a rundown of exactly what the “embedded SIM” in the new 9.7-inch iPad Pro means for you:
All iPad Pro 9.7″ devices have a SIM slot right on the exterior and you can put another carrier’s SIM in that slot even if the iPad Pro itself has been locked to AT&T. In other words, the internal SIM may be locked, but you can “switch” carriers by using another physical SIM that you buy. [emphasis in original]
So, there are two SIMs in the iPad: Apple’s own, embedded within, and the swappable SIM. At some point, I’d imagine that Apple would like its embedded SIM to be the one and only, with all band switching handled by hardware, but it’s probably going to need to do some convincing where the carriers are concerned (as well as expanding the Apple SIM compatibility).