The source – developers.google.com
The tag is designed to solve the problem when website owners want their content to be indexed on other people's resources.
It allows content embedded via <iframe (https://developer.mozilla.org/docs/Web/HTML/Element/iframe)> to be indexed by Google. You can use this tag to tell Google that you want content indexed in HTML tags (such as object), even if the pages have a noindex tag.
You can use in the meta tag:
<meta name="googlebot" value="noindex,indexifembedded" />
Or in X-Robots-Tag: