private void considerCopyTransformForExtraSummaryField(Schema schema, SummaryField field) {
        if (field.getTransform() == SummaryTransform.NONE && field.hasExplicitSingleSource() &&
                fieldIsExtraSummaryField(schema, field.getName())) {
            field.setTransform(SummaryTransform.COPY);
        }
    }
This processor creates a {@link com.yahoo.schema.document.SDDocumentType} for each {@link Schema}
object which holds all the data that search
associates with a document described in a search definition file. This includes all extra fields, summary fields and
implicit fields. All non-indexed and non-summary fields are discarded.
config-model/src/main/java/com/yahoo/schema/processing/AddExtraFieldsToDocument.java