Question
The rule "DHCP Client", that allows 68/tcp(iptables) and 546/tcp(ip6tables) appeared in the Plesk Firewall after Plesk 18.0.38, why?
Answer
The rule was added in order for the IPv6 addresses to be assigned properly via DHCP on the cloud-hosted servers.
Note: This rule can be safely disabled or altered at discretion in case the server doesn't use IPv6 via DHCP.