The Iteration operation 'each' works as a simple search-results over a space and a regular expression.
The space of the control is used to identify the space over which the search takes place. The value of the control will be used as a search pattern. Not all spaces are supported for the each operation - namespace,grammar,parm,url are those that are currently (2010) not supported. Of those, url cannot be supported.
The meta-field #key is used to identify the value of the current found (matching) result within the operation.
This is an example finds a list of all files matching the regex 'boo.\.xml' in the current working directory, and lists them.
<iteration space="each">
<control space="file" value="boo.\.xml" />
<body>[<s space="field" value="#key" />]</body>
</iteration>[book.xml][bool.xml]