Paste the full headers of an email and we walk its ARC chain — the chain of ARC-Seal, ARC-Message-Signature, and ARC-Authentication-Results headers added by every forwarder / mailing list along the path. ARC (RFC 8617) lets the final receiver trust an upstream authenticator's verdict even when SPF and DKIM have been broken by forwarding.
SPF breaks the moment a forwarder rewrites the envelope. DKIM body-hashes break when a list reformats the body. Without ARC, the final receiver sees both fail and either rejects the message (if you're at p=reject) or quarantines it (at p=quarantine). With ARC, the receiver can see "Gmail saw this pass DKIM at hop 1 before forwarding it" and accept the message based on that upstream verdict.
ARC adoption is high among major receivers (Google, Microsoft, Yahoo all add ARC headers) and most modern mailing-list managers. If a message you'd expect to pass DMARC is failing, the ARC chain often tells you which hop introduced the problem.