When a contact clicks on the Preferences link in the email, and there are categories for them to select, whichever categories they currently belong to will be checked by default. This is to help the contact know what newsletter(s) they currently receive.
What happens if a contact unchecks a box?
If a contact wishes to not receive correspondence for a specific category, the contact can uncheck the box and update their records. In the contact's subscription info, it will show that the contact is unsubscribed from that category with the date it was unsubscribed.
Alternatively, contacts are able to check any unchecked boxes should they like to start receiving campaigns for a different category. The contact's category page will then show that contact's dates in their Date Joined field.
What action proceeds the other? Importing into a category or a contact updating their categories?
Contacts who unsubscribe themselves from a category via your preference center cannot be added back to that category via import. Contacts who unsubscribed via your preference center will need to either opt back in via your preference center, or be manually added back to the category.