Hello Liz, great to see you back.
I think the, "British Possessions" on Catherine’s passenger list means…..
Australia comes under, "British Possessions" (Lands under British Rule…so is New Zealand).
Catherine’s passport allows her to travel between England and Australia.
In case Catherine needed to travel back home.
Here is a wonderful Australian (Victoria) website that you can e-mail and ask them any questions you need answered.
Good luck…and if you need any more help for anything at all, just e-mail me.
www.museumvictoria.com.au/discoverycentre/infosheets/passenger-lists-on-the-internet/
Averil