The "Quit" button quits the editor. If changes have been made since the last save (either a "Save Contig" or an auto- save) you will be asked whether you wish to save these changes. Answering "Cancel" abandons the quit process and provides control of the editor again, otherwise the appropriate action will be taken and the editor quitted.
Within a join editor, the "Quit" button is changed to "Join/Quit". Pressing it will prompt for making the join. You will be told the percentage mismatch of the overlapping consensus sequences. The join can either be accepted, rejected, or cancelled (in which case the editor is not quitted and the join is not made).