• In the interface that you tag with this attribute, implement methods that perform test object operations on the control.
• Each method you design must have the same signature as the test object operation that it implements (as defined in the test object configuration file).
• Implement methods for any new test object operations you defined for your test object class, or if you want to override the operation implementation inherited from the base class.
• A custom server class cannot implement more than on interface tagged with RunInterfaceAttribute.

System.Object
   System.Attribute
      Mercury.QTP.WPF.CustomServer.RunInterfaceAttribute

Reference