L2TPv3 over IPv6 is used to establish L2TPv3 tunnels on an IPv6 public network, so that Layer 2 user packets can be transparently transmitted across the IPv6 public network.
L2TPv3 over IPv4 is used to establish L2TPv3 tunnels on an IPv4 public network, so that Layer 2 user packets can be transparently transmitted across the IPv4 public network.