An authenticator that implements CTAP2 is called a FIDO2 authenticator (also called a WebAuthn authenticator). If that authenticator implements CTAP1/U2F as well, it is backward compatible with U2F. A YubiKey 5 Series security key can support both CTAP 1 and CTAP 2 which means it can support both U2F and FIDO2 and deliver strong single factor (passwordless), strong two-factor and strong multi-factor authentication.