Volunteer source is normally used to indicate how you first acquired a volunteer for your organization. By recording that value when the volunteer is first entered, you can run reports later to determine where you get your best volunteers and know where to focus your recruitment efforts. A person may have only one volunteer source. If you already have a section on your application asking how a volunteer found you, you would want the choices here to mirror those options.
Someone who is not a volunteer is still allowed to have a volunteer source.
Deleting a volunteer source removes that volunteer source from all people who currently have it.
Some VSys customers have asked about using this field for other data. Since you can define all the menu options that appear here, use it however it is useful for you!