In the source window, you can load, create, edit, and save FORTH source files.
From here you can also send (upload) source files to the MCU.
Source files can be nested with the INCLUDE directive. There is a separate page descriping this handy feature.
You can send the whole source file, or mark selected text in the file, and send the selected text only.
There is a handy search facility - usable if you are dealing with bigger source files.