When creating an DHCPv6-PD configuration, it should be enough to specify the length and it would ask for the delegation from DHCPv6 server without the need to assign it to any interface.

It should have route added for the delegated prefix and it would be useful when trying to sub-delegate or route the prefix to an downstream router.

Currently, the following configuration will result in an error DHCPv6-PD requires an interface where to assign the delegated prefix:

interfaces {
     ethernet eth0 {
         address dhcp
     }
     ethernet eth1 {
         address dhcpv6
         description WAN
         dhcpv6-options {
             pd 0 {
                 length 56
             }
         }
         ipv6 {
             address {
                 autoconf
             }
         }
     }