If the user already has a record, then the Opt in checkbox will only update the "Accepts Email" field on the user's record if the checkbox is marked to opt-in and if the constituent is logged in. This will make the "Accept Email" field on their constituent record change to "Yes".

If the user had opted in to accepting email previously, and then filled out a survey with the Opt in checkbox unmarked, they would not be opted out of accepting site wide email. The Accept Email field on their constituent record in that case, would remain "Yes". The same applies to postal mail. 

For a user to opt-out of email after being opted in already, they would need to click an Unsubscribe link in an email, or login to their UserCenter and adjust their Email Preferences.