Mostly needed to reach the outside with IPv6 using an IPv6-enabled router on your link.
7.2.2. Using "route"
# /sbin/route -A inet6 add <ipv6network>/<prefixlength> gw
¬ <ipv6address> [dev <device>]
A device can be needed, too, if the IPv6 address of the gateway is a link local one.
Following shown example adds a route for all currently global addresses (2000::/3) through gateway 3ffe:ffff:0:f101::1
# /sbin/route -A inet6 add 2000::/3 gw 3ffe:ffff:0:f101::1