[JKT]

The reason you are confused by the Shared is probably because you only have a home address entered in your Address Book entry. If you have e.g. a work address, etc. entered as well it would become clear that the Shared information is available to all your addresses in the Autofill pane. With multiple addresses, you can select e.g. your Work entry in the Autofill pane to have your work details entered by autofill rather than your Home entry details. The shared information is e.g. all the text entries you have auto-saved.