Windows includes the ability to associate file extensions with specific applications. For example, a graphics program might be associated with files with a .JPG extension, while Notepad could be associated with files with a .TXT extension.
When you attempt to start an application from the command line or a batch file, TCC first searches for an external program file with a standard extension (.EXE, .CMD, etc.). It then checks executable extensions. If all of these tests fail, TCC passes the command name to Windows to see if Windows can find an association for it.
TCC offers two commands which provide control over file associations. Both should be used with caution to avoid creating errors in the registry or damaging existing file types. The ASSOC command modifies or displays the associations between extensions and file types in the Windows registry. The FTYPE command modifies or displays the default command used to "open" a file of a specified type.
Executable extensions defined in TCC always take precedence over file associations defined in Windows. For example, if you associate the .TXT extension with your own editor using a TCC executable extension, and Windows has associated .TXT with Notepad, your setting will have priority, and the association with Notepad will be ignored when you invoke a .TXT file from within TCC.