--- a/src/doc/user/usermanual.xml
+++ b/src/doc/user/usermanual.xml
@@ -6063,7 +6063,10 @@
<varlistentry>
<term>[prefixes]</term>
<listitem><para>A field becomes indexed (searchable) by having
- a prefix defined in this section.</para>
+ a prefix defined in this section. There is a more complete
+ explanation of what prefixes are in used by a standard recoll
+ installation. In a nutshell: extension prefixes should be all
+ caps, begin with XY, and short. E.g. XYMFLD.</para>
</listitem>
</varlistentry>
<varlistentry>