Collections in Plone are the most powerful tool content editors and site managers have to construct navigation and site sections.

This is a brand new implementation of collections for Plone, using ajax/javascript to make a simpler, easier and streamlined user experience for using collections. Having a more lightweight backend that does not depend on many nested criteria types.

It's designed with simplicity and usability as a main focus, so content editors and site managers can create complex search queries with ease.

plone.app.collection and (or more precisely the underlying plone.app.querystring) uses plone.app.registry records to define possible search criteria for a collection.

If you want to add your own criteria, say to choose a value from a custom index, you have to create a plone.app.registry record for this index in your generic setup profile (e.g profiles/default/registry.xml):

<registry>
  <records interface="plone.app.querystring.interfaces.IQueryField"
           prefix="plone.app.querystring.field.department">
    <value key="title">Department</value>
    <value key="description">A custom department index</value>
    <value key="enabled">True</value>
    <value key="sortable">False</value>
    <value key="operations">
        <element>plone.app.querystring.operation.string.is</element>
    </value>
    <value key="group">Metadata</value>
  </records>
</registry>

The title-value refers to the custom index ("Department"), the operations-value is used to filter the items and the group-value defines under which group the entry shows up in the selection widget.