Please see this KB article for a discussion on what IP forwarding means:
You have an IPv6 deployment or a mixed IPv4 and IPv6 environment and you want to enable IPv6 forwarding.
IPv6 forwarding can be enabled only via CLI and must be enabled separately.
To enable IPv6 forwarding, open a secure shell or telnet connection to the proxy or connect via serial line then execute the following commands:
ProxySG#(config)ipv6 forwarding enable