If Command Extensions are enabled, external command invocation through the command line or the START command changes as follows: ...

If Command Extensions are enabled, external command invocation
through the command line or the START command changes as follows:

non-executable files may be invoked through their file association just
    by typing the name of the file as a command.  (e.g.  WORD.DOC would
    launch the application associated with the .DOC file extension).
    See the ASSOC and FTYPE commands for how to create these
    associations from within a command script.

When executing an application that is a 32-bit GUI application, CMD.EXE
    does not wait for the application to terminate before returning to
    the command prompt.  This new behavior does NOT occur if executing
    within a command script.

When executing a command line whose first token is the string "CMD "
    without an extension or path qualifier, then "CMD" is replaced with
    the value of the COMSPEC variable.  This prevents picking up CMD.EXE
    from the current directory.

When executing a command line whose first token does NOT contain an
    extension, then CMD.EXE uses the value of the PATHEXT
    environment variable to determine which extensions to look for
    and in what order.  The default value for the PATHEXT variable
    is:

        .COM;.EXE;.BAT;.CMD

    Notice the syntax is the same as the PATH variable, with
    semicolons separating the different elements.

When searching for an executable, if there is no match on any extension,
then looks to see if the name matches a directory name.  If it does, the
START command launches the Explorer on that path.  If done from the
command line, it is the equivalent to doing a CD /D to that path.