Mapping is the process of performing transformations to resolve data representation differences between services or document formats. By linking variables to each other on the Pipeline tab, you can accomplish name transformations and structural transformations. However, to perform value transformations you must execute some code or logic. 
Developer provides two ways for you to invoke services: You can insert INVOKE steps or you can insert transformers onto the Pipeline tab. Transformers are the services you use to accomplish value transformations on the Pipeline tab.