Matthew Panzarino, after explaining how the embedded Apple SIM in the new iPad Pro works:

That might not sound like great news, but there is one very bright spot for anyone who wants to switch carriers later. 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.

I was wondering how this worked. Good to know.