diff --git a/man/sd-id128.xml b/man/sd-id128.xml
index 891a4585c6..b6e50a559e 100644
--- a/man/sd-id128.xml
+++ b/man/sd-id128.xml
@@ -90,7 +90,10 @@
     <programlisting>#define SD_MESSAGE_COREDUMP SD_ID128_MAKE(fc,2e,22,bc,6e,e6,47,b6,b9,07,29,ab,34,a2,50,b1)</programlisting>
 
     <para><constant>SD_ID128_NULL</constant> may be used to refer to the 128-bit ID consisting of only
-    <constant>NUL</constant> bytes.</para>
+    <constant>NUL</constant> bytes (i.e. all bits off).</para>
+
+    <para><constant>SD_ID128_ALLF</constant> may be used to refer to the 128-bit ID consisting of only
+    <constant>0xFF</constant> bytes (i.e. all bits on).</para>
 
     <para><function>SD_ID128_MAKE_STR()</function> is similar to <function>SD_ID128_MAKE()</function>, but creates a
     <type>const char*</type> expression that can be conveniently used in message formats and such:</para>