Index: /trunk/source/doc/src/using.xml
===================================================================
--- /trunk/source/doc/src/using.xml	(revision 14684)
+++ /trunk/source/doc/src/using.xml	(revision 14685)
@@ -1482,4 +1482,10 @@
     <varname>vector</varname> where the decoding ended.
   </para>
+  <para>
+    Sequences of octets in <varname>vector</varname> that cannot be
+    decoded into characters according to
+    <varname>external-format</varname> will be decoded as
+    #\Replacement_Character.
+  </para>
   </refsect1>
 </refentry>
@@ -1532,4 +1538,11 @@
     If <varname>vector-offset</varname> is supplied, data will be written
     into the output vector starting at that offset.
+  </para>
+  <para>
+    Characters in <varname>string</varname> that cannot be encoded
+    into <varname>external-format</varname> will be replaced with an
+    encoding-dependent replacement character
+    (#\Replacement_Character or #\Sub) before being encoded and written
+    into the output vector.
   </para>
   </refsect1>