This toggles the underlining of (possible) spelling mistakes. Default: ticked (on).
This also sets the document-level default for the system attribute $NoSpelling. For the above default, the $NoSpelling default is false.
On low-powered systems and/or with documents with large amounts of $Text leaving this off may help with performance. Otherwise, leaving the setting turned off simply avoids the red underline spelling prompts. The latter are not helpful if showing the app in a presentation or if using multiple languages in $Text.
See also—notes linking to here: