While the introduction of what amounts to a universal SIM card in the new iPad Air 2 and iPad mini 3 brought on celebration of an impending future of easier carrier switching with just one SIM, we're only halfway there. Not only does the SIM not support Verizon, but if you try to activate it on AT&T, it will lock the SIM to that carrier.
Using Apple SIM, you can choose from different cellular carriers and their various programs. The data plans vary by carrier. For instance, in the United States, you can choose a domestic plan from either Sprint or T-Mobile and also pick an alternate plan from the other carrier as needed. When you choose AT&T on iPad Air 2 and iPad mini 3, AT&T dedicates Apple SIM to their network only.
Naturally, T-Mobile CEO John Legere used this as a moment to poke at his rivals, noting that activating an Apple SIM-equipped iPad on his magenta network won't result in any carrier locking. If all else fails and your iPad's Apple SIM gets locked to AT&T or another carrier, never fear — the iPad itself is still unlocked. Swap it out with another SIM, even a new Apple SIM from Apple, and everything will be back to freedom.