Hi,
When you go and buy the phone, you can either take with you 2 SIM cards, one from Verizon and the other from a different network. Once you buy the phone and the person from the apple store let's you know that the phone is unlocked, you can try first the Verizon SIM and then the other SIM.
The order is very important, because the phone may be locked under two different policies when you buy it directly from apple. The first one and the best one is Multi-Mode Unlock = phone is unlocked.
The second one, is if in some case one of the guys working there is not doing his/her job correctly, is for the phone to be locked to US Reseller Flex Policy = first SIM you put inside the phone, the phone get's locked to that network.
The reason why it is so important to use the Verizon SIM is because Verizon SIM are already unlocked and if you place it into a phone that is locked unde US Reseller Flex Policy, it will get locked under Verizon, but since their policy is unlocked the phone will be Verizon SIM unlocked and you will be able to use other CDMA and GSM networks.
Iphone 7 and 7+ have the band width to work on both CDMA or GSM networks. The Band width applies only to iPhone models up to 6 plus.
Hope this helps and I hope I have made everything a little clearer!