Microsoft Knowledge Base Article 926464 contains the following Introduction:
A new version of the Microsoft Windows Update (WU) offline scan file has been released. This new offline scan file is available in addition to the existing WU offline scan file, Wsusscan.cab. The new offline scan file, Wsusscn2.cab, has a new format.
You must update any scanning solution that uses the existing offline scan file by March 2007 so that it uses the new CAB file. Microsoft solutions that use the existing offline scan file include Microsoft Systems Management Server Inventory Tool for Microsoft Updates (SMS ITMU), the Microsoft Baseline Security Analyzer (MBSA), and the Windows Update Agent (WUA).
New releases of SMS ITMU, of the MBSA, and of the WUA stand-alone installer have also been released.
Some non-Microsoft solutions that use the existing WU offline scan file for offline security update detection must also be updated.
The WU offline scan file in the previous format will continue to be updated whenever new security bulletins are released. However, older updates will be removed from each subsequently published version of the Wsusscan.cab file so that the file stays within the size limits.
By March 2007, the existing WU offline scan file will no longer be maintained. At that time, customers will have to use the new Wsusscn2.cab file. Attempts to scan by using Wsusscan.cab will return an error. This error will indicate that the WUA must be upgraded.