This option causes BBAutoComplete use macOS’s built-in spell checker to find additional completions for the text that you’ve typed, after you’ve cycled through all the completions available in the current window and other open documents. This is useful when the open documents don’t contain much text, and therefore aren’t very useful for suggesting completions. It can also be used to help type words that you aren’t quite sure how to spell. One reason to turn this option off would be if you often cycle through the whole list of completions and want to avoid cycling through a potentially long list of suggestions from the spell checker.