The Compose Expression editors are used to qualify the mappings that are already defined in the Mapping view. Use the editors to define more specifically how values are to be assigned and manipulated, and if necessary when they should, or should not, occur. You can also add other fields and apply different built-in and user-defined functions where required.
The Compose Expression editor is launched when you double-click a Target value field in the Overview pane. The actual appearance and content of the editor that is opened depends on the type of mapping that is being edited. Each of the mapping types uses its own Compose Expression editor, which contains relevant functions.