Question: What is config.xml?
Answer: The Config.xml file is an optional Windows® User State Migration Tool (USMT) 4.0 file that you can create using the /gen config option with the ScanState.exe tool. If you want to include all of the default components, and do not want to change the default store-creation or profile-migration behavior, you do not need to create a Config.xml file.
However, if you are satisfied with the default migration behavior defined in the MigApp.xml, MigUser.xml and MigDocs.xml files, but you want to exclude certain components, you can create and modify a Config.xml file and leave the other .xml files unchanged. For example, you must create and modify the Config.xml file if you want to exclude any of the operating system settings that are migrated. It is necessary to create and modify this file if you want to change any of the default store-creation or profile-migration behavior (Best Online Training Institute).
The Config.xml file has a different format than the other migration .xml files because it does not contain any migration rules. It contains only a list of the operating system components, applications, user documents that can be migrated, as well as user-profile policy and error-control policy. For this reason, excluding components using the Config.xml file is easier than modifying the migration .xml files, because you do not need to be familiar with the migration rules and syntax. However, you cannot use wildcard characters in this file.