[SR-Users] Why set this flag on registration?

Hello, In the example for a dual-stack IPv6/IPv4 Kamailio setup here: https://kb.asipto.com/kamailio:kamailio-mixed-ipv4-ipv6 the "FLB_IPV6" flag is being sent on IPv6 registrations. Why set this flag on IPv6 registrations? If I understand correctly, registration requests never make it beyond !save("location so it's not like the flag can be used further in the dial-plan. The code in question: route[REGISTRAR] { if (is_method("REGISTER")) { if(isflagset(FLT_NATS)) { setbflag(FLB_NATB); # uncomment next line to do SIP NAT pinging ## setbflag(FLB_NATSIPPING); } if(af==INET6) setbflag(FLB_IPV6); if (!save("location")) sl_reply_error(); exit; } } Any help is appreciated. Thanks