In addition to have a single IPv6 address (/128) Instances in Basic Networking should be able to have a IPv6 subnet, like a /60 for example routed to them.
The mechanism for this is DHCPv6 Prefix Delegation. A DHCPv6 server can tell the Instance which subnet is routed to it.
On the physical router a (static) route needs to be configured to do this. So in Basic Networking it will be up to the network admin to make sure the routes are present.
The Management Server will pick a subnet for a Instance when needed and configure the VR with the proper DHCPv6 arguments so that the right answer is provided to the Instance.
For example when running containers it is very nice to have a subnet routed to your Instance so you can give each container a unique IPv6 address.