Here you can control the way auto-complete acts. You can turn it off by unchecking Complete words as you type. The second option allows you to delete a selection from the auto-completed text and search for the rest. For example if you type 'annual' and the auto-completion brings you 'annual report' the ' report' will be selected so you can delete it or if you have the option for non selected text the program will delete ' report' and search only for 'annual' upon Enter pressed. But if you don't have this option turned on then the search pattern will be auto-completed to 'annual report'.
The option: Activate main window on... is useful when you are using the Special selection windows. When you move the mouse outside them to the main window (and have the option on) it will activate it without the need of a click.