@lhannell yes the user ID is the ID of your account. Since it is possible to change, add a tracker, or give a tracker the serial number needs to stay with the tracker.
So yes every tracker has a serial number and a unique Bluetooth MAC address, i think. I say i think because the Fitbit server uses the MAC to identify the tracker, so it is possible that the two might be the same, but unlikely.
There should be a list of the calls in their Wiki.
I've moved your post to theweb API as @bbarrera suggests.
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