The transformed content that is returned to the repository is JSON and specifies what properties that should be updated on the source node. For example:
{"{http://www.alfresco.org/model/content/1.0}description":"Making Bread",
"{http://www.alfresco.org/model/content/1.0}title":"Making Bread",
"{http://www.alfresco.org/model/content/1.0}author":"Fred"}