Orchestration
Sometimes composers leave the orchestration of their works to other people.  Leonard Bernstein is famous as the composer of West Side Story, but although he composed all the notes he left it to other people to orchestrate.  The composer Maurice Ravel took a piano piece called Pictures at an Exhibition by Modest Mussorgsky (who had died forty years earlier). He arranged it for orchestra.  Most people know this version rather than Mussorgsky’s piano version.