missing documentation: subnet selection by interface.
In the ARM under 8.4. Shared Networks in DHCPv4, it is indirectly shown how interface may be specified at the subnet level as a hint during subnet selection while talking about what not to do. However, in the ARM under 8.6. How the DHCPv4 Server Selects a Subnet for the Client, this is not mentioned.
It is mentioned in DHCPv4 Subnet Selection diagram in the appendix. The corresponding DHCPv6 section on subnet selection (9.2.18. IPv6 Subnet Selection) shows the possibility.
I couldn't find documentation of interface being allowed in DHCPv4 subnet {} block as a hint for subnet selection anywhere else in the ARM.