Finally, support for delayed environment variable expansion has been added. This support is always disabled by default, but ...

Finally, support for delayed environment variable expansion has been
added.  This support is always disabled by default, but may be
enabled/disabled via the /V command line switch to CMD.EXE.  See CMD /?

Delayed environment variable expansion is useful for getting around
the limitations of the current expansion which happens when a line
of text is read, not when it is executed.  The following example
demonstrates the problem with immediate variable expansion:

    set VAR=before
    if "%%VAR%%" == "before" (
        set VAR=after
        if "%%VAR%%" == "after" @echo If you see this, it worked
    )

would never display the message, since the %%VAR%% in BOTH IF statements
is substituted when the first IF statement is read, since it logically
includes the body of the IF, which is a compound statement.  So the
IF inside the compound statement is really comparing "before" with
"after" which will never be equal.  Similarly, the following example
will not work as expected:

    set LIST=
    for %%i in (*) do set LIST=%%LIST%% %%i
    echo %%LIST%%

in that it will NOT build up a list of files in the current directory,
but instead will just set the LIST variable to the last file found.
Again, this is because the %%LIST%% is expanded just once when the
FOR statement is read, and at that time the LIST variable is empty.
So the actual FOR loop we are executing is:

    for %%i in (*) do set LIST= %%i

which just keeps setting LIST to the last file found.

Delayed environment variable expansion allows you to use a different
character (the exclamation mark) to expand environment variables at
execution time.  If delayed variable expansion is enabled, the above
examples could be written as follows to work as intended:

    set VAR=before
    if "%%VAR%%" == "before" (
        set VAR=after
        if "!VAR!" == "after" @echo If you see this, it worked
    )

    set LIST=
    for %%i in (*) do set LIST=!LIST! %%i
    echo %%LIST%%