diff --git a/reference/image/functions/imagexbm.xml b/reference/image/functions/imagexbm.xml
index 3ff2665ff8..2c3cae71d3 100644
--- a/reference/image/functions/imagexbm.xml
+++ b/reference/image/functions/imagexbm.xml
@@ -34,6 +34,13 @@
      <term><parameter>filename</parameter></term>
      <listitem>
       <para>&gd.image.path;</para>
+      <para>
+       The <parameter>filename</parameter> (without the .xbm extension) is also
+       used for the C identifiers of the <acronym>XBM</acronym>, whereby non
+       alphanumeric characters of the current locale are substituted by
+       underscores. If <parameter>filename</parameter> is set to &null;,
+       <literal>image</literal> is used to build the C identifiers.
+      </para>
      </listitem>
     </varlistentry>
     <varlistentry>