The purpose of this mapping is to define an heterogeneous set of fields that belong to some modules. Simply a bunch of fields. This could be used to define a set of fields to launch a mass operation upon or a set of fields which must be excluded/included from some global process.
The accepted format is:
<map> <module> <name>ModuleName</name> <fields> <field> <name>fieldname</name> <info>anything you need</info> </field> ..... </fields> </module> .... </map>
You will be able to get the set of fields using the getFieldSet() method, and the fields on one module with the getFieldSetModule($module) method.