https://www.cnet.com/news/moto-z3-phone-sells-at-verizon-aug-16-with-5g-to-come/

Starting in early 2019, Motorola will release a 5G Moto Mod that will tap into Verizon's 5G network, once the first of the carrier's 5G networks flickers to life. 5G is so crucial because it's the next generation of fast data that will make phones upload and download an order of magnitude faster than the fastest 4G LTE networks today. We're talking about downloading 90 Spotify songs in 10 seconds, at up to 5 gigabits per second. 

Like the original Moto Z Force and Moto Z2 Force before it, the Moto Z3 here will make its carrier home exclusively on Verizon in the US, at least at first.