Use ContentListingFactory to create modifiable items. These new items are used as groups in the custom grouper (see MCLFCustomGrouper). Modifiable item is like MCLFItem but you can add new fields to the modifiable item. One item can contain multiple fields, but normally it is not needed in a custom grouper. The field consists of a Field ID and field data. Type of the field is defined with Field ID. See TCLFDefaultFieldId in CLFContentListing.hrh.