+=head1 VALUE IDENTIFIER
+
+The values in the collectd are identified using an five-tupel (host, plugin,
+plugin-instance, type, type-instance) where only plugin-instance and
+type-instance may be NULL (or undefined). Many functions expect an
+I<%identifier> hash that has at least the members B<host>, B<plugin>, and
+B<type>, possibly completed by B<plugin_instance> and B<type_instance>.
+
+Usually you can pass this hash as follows:
+
+ $obj->method (host => $host, plugin => $plugin, type => $type, %other_args);
+
+=cut
+