Use the following command to check the available devices:
xcrun simctl list devices
What you can do with the above is, while developing the react native apps, you may want to select specific device and IOS version. So, you could use it like:
react-native run-ios --udid="9BAD11E6-0B51-4904-893F-3C12F33B312A"
This way you will have full control for the device to test on.