What does USMT, hence USMTGUI, atually migrate?

The following document is an USMT overview from MS technet pages located here.
https://technet.microsoft.com/en-us/itpro/windows/deploy/usmt-what-does-usmt-migrate

Note:
Due to lacking updates by Microsoft, of both XML files and documentation, the following is to be taken with “a grane of salt”.
Office 2013 and 2016 are now natively supported and I have modified USMTGUI to also include all home versions of Firefox and Chrome.
User Data (MigUser.xml)

This section describes the user data that USMT migrates by default, using the MigUser.xml file. It also defines how to migrate ACLs.

•    Folders from each user profile. When you specify the MigUser.xml file, USMT migrates everything in a user’s profiles including :
     -  My Documents, My Video, My Music, My Pictures, desktop files, Start menu, Quick Launch settings, and Favorites.

•    Folders from the All Users and Public profiles. When you specify the MigUser.xml file, USMT also migrates the following from the
    All Users profile in Windows® XP, or the Public profile in Windows Vista, Windows 7, or Windows 8:
     o    Shared Documents
     o    Shared Video
     o    Shared Music
     o    Shared desktop files
     o    Shared Pictures
     o    Shared Start menu
     o    Shared Favorites

•    File types. When you specify the MigUser.xml file, the ScanState tool searches the fixed drives, collects and then migrates files
    with any of the following file extensions:
    .accdb, .ch3, .csv, .dif, .doc*, .dot*, .dqy, .iqy, .mcw, .mdb*, .mpp, .one*, .oqy, .or6, .pot*, .ppa, .pps*, .ppt*, .pre, .pst, .pub,
    .qdf, .qel, .qph, .qsd, .rqy, .rtf, .scd, .sh3, .slk, .txt, .vl*, .vsd, .wk*, .wpd, .wps, .wq1, .wri, .xl*, .xla, .xlb, .xls*.
   
Note
    The asterisk (*) stands for zero or more characters.

•   Access control lists. USMT migrates ACLs for specified files and folders from computers running both Windows® XP and
   Windows Vista. For example, if you migrate a file named File1.txt that is read-only for User1 and read/write for User2, these settings
   will still apply on the destination computer after the migration.
   
Important
   To migrate ACLs, you must specify the directory to migrate in the MigUser.xml file. Using file patterns like *.doc will not migrate
   a directory. The source ACL information is migrated only when you explicitly specify the directory.
   For example, <pattern type="File">c:\test docs</pattern>.


Operating-System Components (MigApp.xml)

USMT migrates operating-system components to a destination computer from computers running Windows 7 and Windows 8

The following components are migrated by default using the manifest files:
•    Accessibility settings
•    Address book
•    Command-prompt settings
•    *)Desktop wallpaper
•    EFS files
•    Favorites
•    Folder options
•    Fonts
•    Group membership. USMT migrates users’ group settings. The groups to which a user belongs can be found by right-clicking
     My Computer on the Start menu and then clicking Manage.
•    *)Windows Internet Explorer® settings
•    Microsoft® Open Database Connectivity (ODBC) settings
•    Mouse and keyboard settings
•    Network drive mapping
•    *)Network printer mapping
•    *)Offline files
•    *)Phone and modem options
•    RAS connection and phone book (.pbk) files
•    *)Regional settings
•    Remote Access
•    *)Taskbar settings
•    User personal certificates (all)
•    Windows Mail.
•    *)Windows Media Player
•    Windows Rights Management
*) These settings are not available for an offline migration. For more information, see Offline Migration Reference.

Important
This list may not be complete. There may be additional components that are migrated.

Note
Some settings, such as fonts, are not applied by the LoadState tool until after the destination computer has been restarted.
For this reason, restart the destination computer after you run the LoadState tool.

Supported Applications
Although it is not required for all applications, it is good practice to install all applications on the destination computer before restoring the user state. Installing applications before migrating settings helps to ensure that the migrated settings are not overwritten by the application installers.
Note
The versions of installed applications must match on the source and destination computers. USMT does not support migrating the settings of an earlier version of an application to a later version, except for Microsoft Office.
Note
USMT migrates only the settings that have been used or modified by the user. If there is an application setting on the source computer that was not touched by the user, the setting may not migrate.


Applications settings migrated:

When you specify the MigApp.xml file, USMT migrates the settings for the following applications:
Product    Version
Adobe Acrobat Reader    
AOL Instant Messenger    
Adobe Creative Suite    
Adobe Photoshop CS    
Adobe ImageReady CS    
Apple iTunes    
Apple QuickTime Player    
Apple Safari    
Google Chrome    
Google Picasa    
Google Talk    
IBM Lotus 1-2-3    
IBM Lotus Notes    
IBM Lotus Organizer    
IBM Lotus WordPro    
Intuit Quicken Deluxe    
Money Plus Business    
Money Plus Home    
Mozilla Firefox    
Microsoft Office    2003, 2007, 2010, 2013, 2016
Microsoft Office Access®    2003, 2007, 2010, 2013, 2016
Microsoft Office Excel®    2003, 2007, 2010, 2013, 2016
Microsoft Office FrontPage®    2003, 2007, 2010, 2013, 2016
Microsoft Office OneNote®    2003, 2007, 2010, 2013, 2016
Microsoft Office Outlook®    2003, 2007, 2010, 2013, 2016
Microsoft Office PowerPoint®    2003, 2007, 2010, 2013, 2016
Microsoft Office Publisher    2003, 2007, 2010, 2013, 2016
Microsoft Office Word    2003, 2007, 2010, 2013, 2016
Opera Software Opera    
Microsoft Outlook Express    (only mailbox file)
Microsoft Project    
Microsoft Office Visio®    
RealPlayer Basic    
Sage Peachtree    
Skype    
Windows Live Mail    
Windows Live Messenger    
Windows Live MovieMaker    
Windows Live Photo Gallery    
Windows Live Writer    
Windows Mail    (Windows 7 and 8)
Microsoft Works    
Yahoo Messenger    
Microsoft Zune™ Software    
What USMT Does Not Migrate

The following is a list of the settings that USMT does not migrate. If you are having a problem that is not listed here, see
Common Issues.

Application Settings

USMT does not migrate the following application settings:

•    Settings from earlier versions of an application. The versions of each application must match on the source and destination
    computers. USMT does not support migrating the settings of an earlier version of an application to a later version, except for
    Microsoft Office.
USMT can migrate from an earlier version of Microsoft Office to a later version.

•    Application settings and some operating-system settings when a local account is created. For example, if you run /lac to create a
    local account on the destination computer, USMT will migrate the user data, but only some of the operating-system settings, such
    as wallpaper and screensaver settings, and no application settings will migrate.
•    Microsoft Project settings, when migrating from Office 2003 to Office 2007 system.

•    ICQ Pro settings, if ICQ Pro is installed in a different location on the destination computer. To successfully migrate the settings of
    ICQ Pro, you must install ICQ Pro in the same location on the destination computer as it was on the source computer. Otherwise,
    after you run the LoadState tool, the application will not start. You may encounter problems when:
    o   You change the default installation location on 32-bit destination computers.
    o   You attempt to migrate from a 32-bit computer to a 64-bit computer. This is because the ICQ Pro default installation directory is
        different on the two types of computers. When you install ICQ Pro on a 32-bit computer, the default location is
        "C:\Program Files\...". The ICQ Pro default installation directory on an x64-based computer, however, is “C:\Program Files (x86)\”
Default Migration Scripts

The User State Migration Tool (USMT) 10.0 is designed so that an IT engineer can precisely define migrations using the USMT .xml scripting language. USMT provides the following sample scripts:

•    
MigApp.XML. Rules to migrate application settings (modified by me regarding Chrome and Firefox)

•    
MigDocs.XML. Rules that use the MigXmlHelper.GenerateDocPatterns helper function, which can be used to automatically find user
     documents on a computer without the need to author extensive custom migration .xml files.

•    
MigUser.XML. Rules to migrate user profiles and user data.
     MigUser.xml gathers everything in a user’s profile and then does a file extension- based search of most of the system for other user data.
     If data doesn’t match either of these criteria, the data won’t be migrated. For the most part, this file describes a "core" migration.

     The following data does not migrate with MigUser.xml:
       o    Files outside the user profile that don’t match one of the file extensions in MigUser.xml.
       o    Access control lists (ACLs) for folders outside the user profile.

•    
Custom.XML added by me: Inactive by default (commented out but easily made active).
     Is preloaded with settings to exclude locale drives.