SiteStat.xml	The SiteStat.xml file is the first file downloaded during an update task. It is used to determine the availability of the repository site. If the Agent can't establish the network connection to the repository site, it closes with the following entries in the Agent log: Error occurred while downloading the file SiteStat.xml Error downloading the file CommonUpdater/SiteStat.xml, nainet GetLastError() = 4   If the file is present and contains a status of Enabled, the Agent continues with the update. If the file is not present or has a status of Disabled, the Agent tries the next repository on the list. If the Agent can't connect to any repository in the list, it records the following entry in the agent log: Unable to find a valid repository   If a replication is in progress to a repository, the status is changed to Disabled to protect file integrity and ensure a matched set of files. If the system is ePolicy Orchestrator (ePO) managed, the CatalogVersion is compared to information received from the ePO server. If the version is too old, VirusScan Enterprise (VSE) does not update from this site.
Catalog.z	The catalog.z file reports which files are currently available on that update site. The Agent compares this information against what is installed on the client system. It downloads only the needed files.
DATInstall.mcs	The .MCS files are secure script files used by the Agent for several tasks. The DATInstall.mcs script installs the .DAT files during the AutoUpdate process.
PkgCatalog.z	The PkgCatalog.z file reports which files are contained within a repository package. The Agent uses this information to determine what must be downloaded.

Which files are downloaded when
Update files can include .DAT files, Extra.DAT files, Engines, and Patches. These files are downloaded as required. The status of the client node and the environment determines the needed files. The environment can range from a repository with only .DAT, Extra.DAT, and Engine files to a repository with a wide range of update and installation packages.

If a node is current on all files in an environment where the repositories only include .DAT, Extra.DAT, and Engine files, the task downloads:

• SiteStat.xml
• catalog.z

If a .DAT update is required, the task downloads: 

• SiteStat.xml
• catalog.z
• DATDet.mcs
• PkgCatalog.z (from the VSCANDAT1000\DAT\0000 folder)
• DATInstall.mcs
• Delta.ini (for the .upd file listing)
• 35 V2 incremental .GEM files
• EngDet.mcs

If an Extra.DAT is available, the task downloads: 

• ExtraDATInstall.mcs
• PkgCatalog.z (from the EXTRADAT1000\ExtraDAT\0000 folder)
• extra.dat

If an Engine is needed, the task downloads: 

• EngInstall.mcs
• PkgCatalog.z (from the VSCANENGINE1000\Engine\0000 folder)
• engmin.zip

Additional Information: 

• A maximum of 35 .GEM files are available for download. If the .DATs are out of date, the complete set of .DATs are downloaded in the dat-xxxx.zip file.
• In an ePO managed environment, a repository can have many other products and related updates. There is a detection script for each of those products.
   

Full list of files:

• SiteStat.xml
• catalog.z
• DATDet.mcs
• PkgCatalog.z
• DATInstall.mcs
• Delta.ini
• Incremental: .GEM files
• dat-xxxx.zip
• EngDet.mcs
• EngInstall.mcs
• engmin.zip
• VSE880Det.mcs (VSE 8.8)