Leave overwrite off to fill blanks only; turn overwrite on to replace thin or inconsistent existing alt text.
If overwrite is off, ALT SEO only generates for images missing alt text (and still respects skip-previously-processed and other filters).
Turn overwrite on when existing text is generic (“image”, filenames) or does not match your new format. Start with a collection so you do not rewrite the whole catalog at once.
Related help article: Read the guide →