question: private static void parseCopyFields ( ParseContext context , FieldMapper fieldMapper , List < String > copyToFields ) throws IOException { if ( ! context . isWithinCopyTo ( ) && copyToFields . isEmpty ( ) == false ) { context = context . createCopyToContext ( ) ; for ( String field : copyToFields ) { ParseContext . Document targetDoc = null ; for ( ParseContext . Document doc = context . doc ( ) ; doc != null ; doc = doc . getParent ( ) ) { if ( field . startsWith ( doc . getPrefix ( ) ) ) { targetDoc = doc ; break ; } } assert targetDoc != null ; final ParseContext copyToContext ; if ( targetDoc == context . doc ( ) ) { copyToContext = context ; } else { copyToContext = context . switchDoc ( targetDoc ) ; } parseCopy ( field , copyToContext ) ; } } }
target:  Creates instances of the fields that the current field should be copied to 