-This value is required. As a special exception, if the "base XPath expression"
-(the argument to the B<XPath> block) returns exactly one argument, then this
-option may be omitted.
+=item B<PluginInstanceFrom> I<PluginInstanceFrom>
+
+Specifies a XPath expression to use for determining the I<plugin instance>. The
+XPath expression must return exactly one element. The element's value is then
+used as I<plugin instance>.
+
+=back
+
+If the "base XPath expression" (the argument to the B<XPath> block) returns
+exactly one argument, then I<InstanceFrom> and I<PluginInstanceFrom> may be omitted.
+Otherwise, at least one of I<InstanceFrom> or I<PluginInstanceFrom> is required.
+
+=over 4