Defines which metadatum is mapped on which metadatum. Note that while the key must be unique it only matters here for postprocessing of the value. The mapped MetadatumContributor has full control over what metadatafield is generated. Defines which metadatum is mapped on which metadatum. Note that while the key must be unique it only matters here for postprocessing of the value. The mapped MetadatumContributor has full control over what metadatafield is generated.