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ISC DHCP has spawning classes:

   Many  cable  modem  head-end  systems  can be configured to add a Relay
   Agent Information option to DHCP packets when relaying them to the DHCP
   server.   These  systems typically add a circuit ID or remote ID option
   that uniquely identifies the customer site.  To take advantage of this,
   you can write a class declaration as follows:

   class "customer" {
     spawn with option agent.circuit-id;
     lease limit 4;
   }

   Now  whenever  a  request comes in from a customer site, the circuit ID
   option will be checked against the class's hash table.  If  a  subclass
   is  found that matches the circuit ID, the client will be classified in
   that subclass and treated accordingly.  If no subclass is found  match-
   ing  the  circuit  ID,  a  new  one  will  be created and logged in the
   dhcpd.leases file, and the client will be classified in this new class.
   Once  the  client  has been classified, it will be treated according to
   the rules of the class, including, in this case, being subject  to  the
   per-site limit of four leases.

   The  use  of the subclass spawning mechanism is not restricted to relay
   agent options - this particular example is given only because it  is  a
   fairly straightforward one.

The important statement is the manual is:

The reason that spawning classes were created was to make it possible to create lease-limited classes on the fly.

So they are very bound to the lease limitation feature. As Kea does not support it in fact the whole concept of spawning classes is useless for Kea...