The Source field is only used for documents. It records the location of the original item. This might be an archival collection, a journal or newspaper, conference proceedings, or other published book.
Use complete citations whenever possible. If the item is available online, add a URL link to it.
Margaret Sanger, ed.. International Aspects of Birth Control (New York, 1925), pp. 45-49.
Margaret Sanger Papers, Library of Congress.
Birth Control Review, June 1932, pp. 10-13.
New York Times, Aug. 15, 1921.