MDAC 2.5 SDK - OLE DB Programmer's Reference
Chapter 1: Overview of OLE DB
In addition to the return codes and status information returned by each method in OLE DB, providers can expose OLE DB error objects containing extended error information, such as a description of the error or the appropriate SQLSTATE. Error objects can be created by any interface on any OLE DB object. Providers can also choose to define custom error objects.