(See answers.yahoo.com)

Yahoo supports these roles:
Knowledge Partner: Specific groups that have been identified by Yahoo
Official: Somebody validated by the Yahoo staff as being authenticated (used, e.g., for celebrities)
Staff: Yahoo answers staff
Top Contributor: (This is less an official role and more a privllege earned)

http://help.yahoo.com/l/us/yahoo/answers/badges/badges-02.html