What is compared
Both domains go through the same live lookup as the full domain report. The comparison then lines up, row by row: registrar, creation and expiry dates, status codes and the registry's DNSSEC flag; nameservers and the A, AAAA and CAA records; MX hosts, detected mail provider, SPF and DMARC records; the network (AS number), allocation country and CDN edge of the addresses, the Server header and the final URL; and the certificate issuer, its expiry date, the TLS version and HSTS.
Each row carries a label with an icon: same, different, or could not be checked. Lists are compared without regard to order or letter case, so two domains with the same nameservers in a different order count as the same.
What it is good for
Before a migration: compare the old domain with the new one and see at a glance which records have not been copied. After a purchase: check whether a name you bought still points at the seller's nameservers or mail. For a brand with several country domains: find the one that has no DMARC record or a certificate from a different issuer. And for a suspicious look-alike name: see whether it shares infrastructure with the original or sits somewhere else entirely.
Limits
A difference is an observation, not a fault: two domains may be set up differently on purpose. "Could not be checked" on either side means the row has no verdict, because a value that could not be read is not the same as a value that is absent. Shared infrastructure does not prove shared ownership, since thousands of unrelated domains use the same DNS and hosting providers. Comparison pages are not indexed by search engines.