This is like ARP populate for IPv4. When it is turned on, an SR OS needs to populate the link-layer address to IPv6 address mapping into the neighbor database based on the DHCPv6 lease information received.
If the IPv6 address of the host does not belong to the subnets of the interface, such a neighbor record should not be created. This could happen when there is a downstream DHCPv6 relay router or prefix delegation requesting router.