The S/N (serial number) is the serial number of your device.
You can find it on the back of the packaging of your device or the back of your ZeWatch4/4HR.
The D/N (device number) is the device number associated to your account.
You can find it in the Settings menu of your ZeWatch4/4HR. Select to see it.
You can also find it in the Settings tab of the ZeWatch4 application. Then, click on the help section menu.
The D/N is associated with your email address. You must unpair your ZeWatch4/4HR from your account through the app to connect your ZeWatch4/4HR with another account.