this is described in more detail in the ConfD user guide and/or man page, relevant to confd_data_reply_value_array() procedure, more specifically, XML STRUCTURES / Value Array sub-sections:
List nodes use a single array element, with type C_NOEXISTS (value ignored), regardless of the actual number of entries or their contents.
As a response to get_object() for “protocol” entry, we just need to tell ConfD there’s some list as a potential content of the object we’re returning - and it’s up to ConfD to get the specific nested contents as necessary (again, invoking further data provider callbacks).