What is the "Source" column in the export of my contacts?

The Source column indicates how the contact was added to your account:

• Import - a contact created via an import
• Manual - a contact created indivdually via Add Contact option
• Survey - a contact created as a result of a survey action
• Preference Center - a contact created via sign up form or other preference center activity
• [Integration Name] - a contact created via integration, such as Salesforce, MyDesktop, or your company's custom integration
• Unknown - generally this indicates the contact was created prior to 1 Sep 2013, when this tracking feature became available