What is the difference between 'maintain records' and 'update records' when adding data?

When importing contacts into a new or existing list, Little Green Plane will ask you whether you wish to maintain or update the records that you are importing.

Selecting the ‘update records’ option when uploading a contact list will override any existing information you may have already imported. ‘Maintain records’ and is set as the default, as it will keep any existing data and simply add to this.