When you configure a signature on Scribe, there is a lot of URLs in it (logo, profile pictures, icons, website address, ...).
To be able to track the conversion rate from your signatures marketing elements, all those URLs domains are shortened with a default domain belonging to us. This domain is sendassets.io.
You can keep this domain but this is not optimal. Spam filters are not happy when they see an email with a lot of URLs pointing to a different domain than the sender email domain.
That's why we give you the opportunity to override this default domain with a subdomain of your main domain. This subdomain is called a sending domain.
So, if you send an email with your address firstname.lastname@example.org you can configure a sending domain like email-signature.my-company.com so that spam filters are happy.
You can find how to configure a DNS in this article.