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-rw-r--r--doc/develop/distro.rst7
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diff --git a/doc/develop/distro.rst b/doc/develop/distro.rst
index 1d2f9c4c32b..01efce40a29 100644
--- a/doc/develop/distro.rst
+++ b/doc/develop/distro.rst
@@ -92,6 +92,13 @@ That said, we have some differences to these documents, namely:
* If ``-`` is passed as fdt argument and ``CONFIG_SUPPORT_PASSING_ATAGS`` is
enabled, then no device tree will be used (legacy booting / pre-dtb kernel).
+* The ``append`` string may use environment variables. For example, an
+ A/B boot setup could use ``append root=PARTLABEL=root_${bootslot}``
+ to set the root filesystem to the right one for the selected slot,
+ assuming the ``bootslot`` environment variable is set before the
+ extlinux.conf file is processed, and the partition is labeled to
+ match.
+
See also doc/README.pxe under 'pxe file format'.
One example extlinux.conf generated by the Fedora installer is::