The Discovery Filter is a specialized MetadataFilter which specifies a whitelist or blacklist an include or exclude list to apply to the entities in the metadata for purposes of the JSON discovery feed optionally produced by the plugin. Any entities not in the feed are still included in all other uses of the metadata.
The type attrriute is constrained to be either IncludeMetadataFilter or ExcludeMetadataFilterInclude or Exclude (prior to V3.2, these were Whitelist and Blacklist, now deprecated).