Explanation for IP.Pages template key values

[Storyteller]

Could someone please be so kind an give me an explanation for the 4 template key values (second parameter) in IP.Pages custom fields: 

Quote

The field key can be used in HTML templates to fetch a specific field, for instance: {$record->customFieldDisplayByKey('my_key', 'listing')|raw}

The key must be unique for this database. If omitted, a key will be automatically generated. Valid values for the second parameter include 'listing', 'display', 'raw', and 'processed'.

Thanks in advance!