diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
index e72dea9..c6bb0be 100644
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -438,8 +438,10 @@
      * Set the :attr:`~sys.flags.dev_mode` attribute of :attr:`sys.flags` to
        ``True``
 
-   * ``-X utf8`` enables the UTF-8 mode, whereas ``-X utf8=0`` disables the
-     UTF-8 mode.
+   * ``-X utf8`` enables UTF-8 mode for operating system interfaces, overriding
+     the default locale-aware mode. ``-X utf8=0`` explicitly disables UTF-8
+     mode (even when it would otherwise activate automatically).
+     See :envvar:`PYTHONUTF8` for more details.
 
    It also allows passing arbitrary values and retrieving them through the
    :data:`sys._xoptions` dictionary.
@@ -789,14 +791,16 @@
 .. envvar:: PYTHONCOERCECLOCALE
 
    If set to the value ``0``, causes the main Python command line application
-   to skip coercing the legacy ASCII-based C locale to a more capable UTF-8
-   based alternative.
+   to skip coercing the legacy ASCII-based C and POSIX locales to a more
+   capable UTF-8 based alternative.
 
-   If this variable is *not* set, or is set to a value other than ``0``, and
-   the current locale reported for the ``LC_CTYPE`` category is the default
-   ``C`` locale, then the Python CLI will attempt to configure the following
-   locales for the ``LC_CTYPE`` category in the order listed before loading the
-   interpreter runtime:
+   If this variable is *not* set (or is set to a value other than ``0``), the
+   ``LC_ALL`` locale override environment variable is also not set, and the
+   current locale reported for the ``LC_CTYPE`` category is either the default
+   ``C`` locale, or else the explicitly ASCII-based ``POSIX`` locale, then the
+   Python CLI will attempt to configure the following locales for the
+   ``LC_CTYPE`` category in the order listed before loading the interpreter
+   runtime:
 
    * ``C.UTF-8``
    * ``C.utf8``
@@ -804,21 +808,32 @@
 
    If setting one of these locale categories succeeds, then the ``LC_CTYPE``
    environment variable will also be set accordingly in the current process
-   environment before the Python runtime is initialized. This ensures the
-   updated setting is seen in subprocesses, as well as in operations that
-   query the environment rather than the current C locale (such as Python's
-   own :func:`locale.getdefaultlocale`).
+   environment before the Python runtime is initialized. This ensures that in
+   addition to being seen by both the interpreter itself and other locale-aware
+   components running in the same process (such as the GNU ``readline``
+   library), the updated setting is also seen in subprocesses (regardless of
+   whether or not those processes are running a Python interpreter), as well as
+   in operations that query the environment rather than the current C locale
+   (such as Python's own :func:`locale.getdefaultlocale`).
 
    Configuring one of these locales (either explicitly or via the above
-   implicit locale coercion) will automatically set the error handler for
-   :data:`sys.stdin` and :data:`sys.stdout` to ``surrogateescape``. This
-   behavior can be overridden using :envvar:`PYTHONIOENCODING` as usual.
+   implicit locale coercion) automatically enables the ``surrogateescape``
+   :ref:`error handler <error-handlers>` for :data:`sys.stdin` and
+   :data:`sys.stdout` (:data:`sys.stderr` continues to use ``backslashreplace``
+   as it does in any other locale). This stream handling behavior can be
+   overridden using :envvar:`PYTHONIOENCODING` as usual.
 
    For debugging purposes, setting ``PYTHONCOERCECLOCALE=warn`` will cause
    Python to emit warning messages on ``stderr`` if either the locale coercion
    activates, or else if a locale that *would* have triggered coercion is
    still active when the Python runtime is initialized.
 
+   Also note that even when locale coercion is disabled, or when it fails to
+   find a suitable target locale, :envvar:`PYTHONUTF8` will still activate by
+   default in legacy ASCII-based locales. Both features must be disabled in
+   order to force the interpreter to use ``ASCII`` instead of ``UTF-8`` for
+   system interfaces.
+
    Availability: \*nix
 
    .. versionadded:: 3.7
@@ -834,10 +849,56 @@
 
 .. envvar:: PYTHONUTF8
 
-   If set to ``1``, enable the UTF-8 mode. If set to ``0``, disable the UTF-8
-   mode. Any other non-empty string cause an error.
+   If set to ``1``, enables the interpreter's UTF-8 mode, where ``UTF-8`` is
+   used as the text encoding for system interfaces, regardless of the
+   current locale setting.
+
+   This means that:
+
+    * :func:`sys.getfilesystemencoding()` returns ``'UTF-8'`` (the locale
+      encoding is ignored).
+    * :func:`locale.getpreferredencoding()` returns ``'UTF-8'`` (the locale
+      encoding is ignored, and the function's ``do_setlocale`` parameter has no
+      effect).
+    * :data:`sys.stdin`, :data:`sys.stdout`, and :data:`sys.stderr` all use
+      UTF-8 as their text encoding, with the ``surrogateescape``
+      :ref:`error handler <error-handlers>` being enabled for :data:`sys.stdin`
+      and :data:`sys.stdout` (:data:`sys.stderr` continues to use
+      ``backslashreplace`` as it does in the default locale-aware mode)
+
+   As a consequence of the changes in those lower level APIs, other higher
+   level APIs also exhibit different default behaviours:
+
+    * Command line arguments, environment variables and filenames are decoded
+      to text using the UTF-8 encoding.
+    * :func:`os.fsdecode()` and :func:`os.fsencode()` use the UTF-8 encoding.
+    * :func:`open()`, :func:`io.open()`, and :func:`codecs.open()` use the UTF-8
+      encoding by default. However, they still use the strict error handler by
+      default so that attempting to open a binary file in text mode is likely
+      to raise an exception rather than producing nonsense data.
+
+   Note that the standard stream settings in UTF-8 mode can be overridden by
+   :envvar:`PYTHONIOENCODING` (just as they can be in the default locale-aware
+   mode).
+
+   If set to ``0``, the interpreter runs in its default locale-aware mode.
+
+   Setting any other non-empty string causes an error during interpreter
+   initialisation.
+
+   If this environment variable is not set at all, then the interpreter defaults
+   to using the current locale settings, *unless* the current locale is
+   identified as a legacy ASCII-based locale
+   (as descibed for :envvar:`PYTHONCOERCECLOCALE`), and locale coercion is
+   either disabled or fails. In such legacy locales, the interpreter will
+   default to enabling UTF-8 mode unless explicitly instructed not to do so.
+
+   Also available as the :option:`-X` ``utf8`` option.
+
+   Availability: \*nix
 
    .. versionadded:: 3.7
+      See :pep:`540` for more details.
 
 
 Debug-mode variables