Discover Bus in Australia, page-21

Thanks @Inductor - Makes sense, nice to have learnt something new, again.

You said: With things only allowed to register once, the failure of a gateway means the failure of the entire personal area network. I think this is probably just a concession to facilitate the development of IoT, and that changes will be ratified in the near future

Looks like this area of the 'ADRC Overview Guide' is referencing just that.

4.3 Future services
4.3.1 Multiple personal area networks (PANs)
Currently, each IoTG has one personal area network (PAN) associated with it.
In the future, one IoTG will be able to have multiple PAN ids. This will enable Things connected to one IoTG
to be set up in different networking groups, each with its own PAN id. This will be particularly useful for a
busy commercial IoTG, allowing a user to send a single message to a group of Things instead of sending
a separate message to each Thing. For example, a user could send a single message to switch off lights in
multiple locations in an office building.