When you add your own sending domain in Settings > Email, the DNS records now show every field in full: Name, Value and, for the MX record, Priority, each with its own copy button. Before, the name was cut off on screen and only the value could be copied, so the long DKIM names had to be retyped.
Copy all records puts the whole set on the clipboard as labelled text, for a note or for whoever runs your DNS.
If your domain's DNS runs at GoDaddy or Namecheap, the names are shown the way those providers' Name field expects them, without your domain at the end.