To highlight features of a thematic layer, the priority should be zero or more. Negative value means that highlighting works in invisible mode—highilight items can be updated but highlighting doesn't applied.

If more than one handler of GraphicSymbolUpdating event is associated with the same thematic layer, updates with higher priority override updates with lesser priority. If two or more updates are applied with the same priority, the latest update is taken into account.

Target Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family, Windows Vista, Windows Server 2008 family

Reference