Authenticated Received Chain (ARC) is an email authentication system that allows an intermediate mail server to sign the original authentication results of an email when forwarding the email. Even if the DMARC rules of the forwarded email are broken for the receiving mail server, it can still be trusted that they were correct when the intermediate server received the email.
ARC signatures are based on trust. It is up to each receiving email server to decide whose signatures it trusts. In the Microsoft O365 environment, trusted ARC signers can be added, for example.
Zone email servers add ARC signatures from the zone.eu domain to forwarded emails. The ARC signature added by Zone is trusted because DMARC etc. checks have been performed before the email is accepted.