diff --git a/documentation/ref-manual/ref-variables.xml b/documentation/ref-manual/ref-variables.xml
index 973a761b95..c7f1122fd4 100644
--- a/documentation/ref-manual/ref-variables.xml
+++ b/documentation/ref-manual/ref-variables.xml
@@ -6073,6 +6073,17 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
                     <link linkend='var-IMAGE_FSTYPES'><filename>IMAGE_FSTYPES</filename></link>
                     variable.
                 </para>
+
+                <para>
+                    The default value of this variable, which is set in the
+                    <filename>meta/conf/bitbake.conf</filename> configuration
+                    file in the
+                    <ulink url='&YOCTO_DOCS_DEV_URL;#source-directory'>Source Directory</ulink>,
+                    is "cpio.gz".
+                    The Linux kernel's initramfs mechanism, as opposed to the
+                    initrd mechanism, expects an optionally compressed cpio
+                    archive.
+                </para>
             </glossdef>
         </glossentry>