
The Referred by field offers a lookup when you fly your cursor over it. If a contact has multiple referrers, the words “Referred by” will be bold-faced, and the primary referrer will appear in this box.
Selecting a primary referrer
Click on the words "Referred by" to bring up the Choose one... dialog box, allowing you to add/modify/delete referrers and select a primary referrer (the one whose name will appear on the MPD tab).

Click on the contact you want to be the primary contact (does not need to be the top on the list) and press OK.
Referred by field
The Referred by name appears twice in the TntMPD database. One field lists only the primary referrer (ReferredBy), while the other one lists all Referrers (ReferredByList).
Lookups. When doing lookups, you can lookup by either field. For example, you can do a lookup only on the Referred by field to find contacts who are the primary referrer. Or, you can do a lookup on the Referred By List field to see all contacts of a referrer.

Page url: http://www.tntware.com/tntmpd/userguides/en/index.html?contacts_referredby.aspx