One of the fields you can use to classify your database contacts is Contact Source. If you would like to designate a Contact Source not already available in your database, you have the ability to add new Contact Sources.
To do so, first go to Manage Your Account:
Then, select Manage Database Lists:
The body of the Manage Database Lists page is controlled by which List you have selected in the Manage Lists column on the right:
After you've clicked Sources, your page will display this list. Below is an example Contact Sources list:
You can add new Contact Sources by clicking the add button in the top right corner. You'll then see the below modal:
Just type in the name for your new Contact Source and click Add Source.
Those Contact Sources that have been manually added to your database have both Edit and Delete options available from their Actions menu, meaning you can edit their name or delete them entirely.
After adding your Contact Source, it will then be available to apply to your contact records. Contact Source is also a searchable field from your Advanced Search page.