Jool & ipv6 only network


i am using JOOL to provide IPv4 gateway to IPv6 only host
it is working as expected with default

jool instance add --pool6 64:ff9b::/96

i like to ask, which rules apply in FW4 for this setup ?
to be clear, host start with IPv6 and end up using IPv4, so it is confusing, when i create FW rule, which address family is used in rules?