Areas are pulled through to new addresses based on the Postcode, and that is controlled in the Area Postcode section of Lookups.
You'll find them in the below location:
Hover on Administration
Click on Lookups
Click on Area Postcodes
Search for an existing entry
You can search for entries using either an existing area or a postcode
Search by Area - Click in the Area box in the top yellow section of the screen and select an area from the menu that appears. When an area is selected click on the Search button.
Search by Postcode - You can enter a partial postcode into the field to search for any matching entries. The minimum is a single character, so for example you can enter "D" and hit the search button to display all entries linked to a postcode starting with "D"
Insert a new Postcode Area link
In the blue section of the Area Postcode screen select the Area that you wish to link to a postcode
In the Postcode section enter the postcode that will be used.
Click the Insert button
The postcode used can be anything down to 2 characters or as high a full postcode, but the more of a postcode you use the more specific the search will be.
Examples:
An entry using "MK" as the postcode will make that area pull though to every address with a postcode starting with "MK".
An entry for "MK20" will pull that area through for all MK20 postcodes only.
An entry for "MK20 4" will pull that are through for postcode starting with "MK20 4" only
Inserting New Postcode Area links en masse
As well as the above option, The "Imports" section of lookups also has an option to import Area Postcode entries en masse.