This feature allows you to configure the Algo server to send outbound traffic through a different external IP address than the one you are establishing the VPN connection with.
Additional info might be found in this issue
Some cloud providers provision a VM with an /128
address block size. This is the only IPv6 address provided and for outbound and incoming traffic.
If the provided address block size is bigger, e.g., /64
, Algo takes a separate address than the one is assigned to the server to send outbound IPv6 traffic.