The Microsoft Download Center Extended Security Update Inventory Tool page contains the following overview:
The SMS Extended Security Update Inventory tool is a scan tool built for the sole purpose of helping customers determine SMS client
computers that may need security updates that are not detectable using the existing SMS Security Update Inventory Tool built on MBSA.
Like the SMS Software Update Inventory tool, this tool also has the instructions for locating each applicable update, downloading it from Microsoft,
and deploying it using SMS.
The SMS Extended Security Update Inventory Tool is built on Enterprise Scan Tool (EST) detection technology.
For more information about the exact detection capabilities of EST and how it differs from MBSA, see Microsoft Knowledge Base Article 894193.
For more information on the SMS Extended Security Update Inventory Tool, please see the included user guide and release notes.