DHCPv6 has an Option Request option that provides a hint to the server of what the client prefers to see. If all possible options were sent from the server to the client, so much information could be sent that some of it would have to be dropped on the way to the client. The server can either use the hint to choose among the options to include in the reply or ignore the hint and choose other items to include. On Oracle Solaris, for example, the preferred options might include the DNS address domain or the NIS address domain but would probably not include the net BIOS server.
The same type of hint is also provided for DHCPv4, but without the special Option Request option. Instead DHCPv4 uses the PARAM_REQUEST_LIST in /etc/default/dhcpagent.