Under "Co-Domains" you can enter subdomains or even total different domains that should point to that web.

Example:

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).