What are Co-Domains?
Under "Co-Domains" you can enter subdomains or even total different domains that should point to that web.
If you can reach your web by www.xyz.tld then you could enter the subdomain www2.xyz.tld under "Co-Domains". You could also enter the new domain abc.tld (e.g. with the host www, so that www.abc.tld points to your web www.xyz.tld).