The editing line contains the expression that you have compiled by means of the various tabs, entered directly or created with Drag & Drop.
The expression is checked continuously as you create it to make sure that the syntax is correct. Any syntax errors are shown in the information pane under the editing line, together with an explanation of the cause of the error. The syntax checker will normally produce an error until the expression is complete. Don't let this worry you. When the expression is complete, the resulting text should be shown with the Designer's sample data.
To make complex expressions clearer, you can split them across several lines with Ctrl+Return. This has no effect on the result.
With the different buttons on the right next to the input field you can
§ mark brackets belonging to the formula expression.
§ mark the expression between matching brackets.
§ edit sum and user variables.
§ undo the last operation.
§ redo the last undo operation.