Microsoft Knowledge Base Article 926464 contains the following introduction:
In November 2006, a new version of the Microsoft Windows Update (WU) offline scan file is being released. This new offline scan file will be 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 will be released at the same time as the new offline scan file Wsusscn2.cab.
Some non-Microsoft solutions that use the existing WU offline scan file for offline security update detection will also have to be updated.
After the new file is released, the existing WU offline scan file will continue to be updated whenever new security bulletins are released. However, older updates will be removed, over time, from each subsequently-published version of the Wsusscan.cab file. This will be done so that the file stays within the size limits.
By March 2007, the existing WU offline scan file will no longer be maintained, and customers will have to use the new Wsusscn2.cab file. At that time, attempts to scan by using Wsusscan.cab will return an error. This error will indicate that the WUA must be upgraded.