This domain, which should be delegating-with-txt-record.carbontxt.org exists to demonstrate how DNS TXT records can be used to "delegate" serving a carbon.txt file to another domain you control, at a URL you choose.
This allows multiple domains hosted by the same organisation or person use the same carbon.txt file, without needing to duplicate the file on each domain.
When setting up the domain, i.e. delegating-with-txt-record.carbontxt.org, you add a TXT record to the domain's DNS records, when you are adding a CNAME, ALIAS or A record. This TXT records points to the desired carbon.txt file, on the actual site run by the organisation responsibile for hosting delegating-with-txt-record.carbontxt.org
In this example, the TXT record for delegating-with-txt-record.carbontxt.org, should be something like:
"carbon-txt=https://used-in-tests.carbontxt.org/carbon.txt 51e9ae1b1b3753083ddd4d1c42a40915171f46916126b9301e5193cedc894ebe"
For more, please see the carbontxt.org website.