Is "domain" a valid directive ?

[Lalatendu Das]

I got an some info regarding the domain directive from bind4 source
base(bind-4.9.10-REL).
in named/ns_init.c file. but it is defined under the LOCALDOM macro
and it is defined in options.h as
#define LOCALDOM      /* permit "domain" directive in named.boot (ucb)
*/
but it is commented in that .h file which means this is not defined at
all. (AFAIK)
So Is this directive is completely depricated.
Even migration script named-bootconf present in bind9 source base is
not generating anything equivalent of this in bind9 configuration
file. So kivndly let me know about this directive. How it is helpful
and when it is used
thanks