RFC 3881 defines this as "Identifier of the source where the event originated."
It should be unique, at least within the Audit Enterprise Site ID. It ties the event to a specific source system.
Target Platforms: Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2