why rbt and rbt64 is checked for slaves?

[kalyanasundaram]

Hi all, 
  I may be asking a very basic question. In the zone.c file the
following conditions are available for slave zones in zone_load. I could
not understand why the rbt and rbt64 is checked?. Can anybody explain
this.

if ((zone->type == dns_zone_slave || zone->type == dns_zone_stub) &&
    (strcmp(zone->db_argv[0], "rbt") == 0 ||
     strcmp(zone->db_argv[0], "rbt64") == 0)) {

I could not get the relevant information from Changlog too. I am sorry.

Thanks,
  -Kalyan