Ok, So I can now answer my own question as the SHS-2320 has arrived today from Amazon.
To use a given tag or NFC Ring with the smart lock, NOTHING is written to the tag.
The supplied tags are blank, the lock simply looks for the UID (Unique Identification Number) of the Ring / Tag.
An example of this is [Card UID: B9 27 4C B5]
A 4 byte value, containing 32 bits and is unique for every tag.
When linking the lower, black tag in my NFCRing to the Smart Lock, the original NDEF message stored in it remains and it will still function as it was originally programmed :-)
So double purpose for the NFC Ring Tag used as the "Door Key"
A standard NDEF message plus a unique key for the lock based on the UID of the NFC tag itself.
This also answers my final point "how easy is it for anyone with a phone to clone my "door key"?!?
Impossible!
Safe and unlocking my house, with my hand.
Most Pleased. dB