Firewall rules with IPv6 suffixes


I use Vyos 1.4 as a router between my ISP and my home network and I was wondering how to best use it as an IPv6 firewall.

The ISP changes the prefix frequently, so I cannot really set a fixed destination address on an IPv6 firewall rule. does mention the ability of using an address suffix instead, but I could not set one in the related field (source). Any idea what the reddit author was talking about?