According to WIKI
returns the ID of the current user if he is running on the server, NULL otherwise.

I assumed that the ID returned corresponds to the RecID in the "sysuser" master.vdb table, and is a constant for the connecting user, i.e.

SELECT RecID, fld_name FROM sysuser WHERE fld_name = 'EMB';
=> 1, EMB

and envisioned using it as a record owner identifier to organize something like a row policy

  CREATE TABLE "test"(

   ...

   "USER_CREATE" USHORT DEFAULT METHOD( 'ifnull( current_user_id(), 0 )' ) NOT NULL,

   "DATE_CREATE" DATE DEFAULT METHOD( 'current_date()' ) NOT NULL );

But the value returned by the function changes randomly from session to session for the same user.

  SELECT current_user_id(), current_user_name();

  => 968, EMB 

  SELECT current_user_id(), current_user_name();

  => 940, EMB 

  SELECT current_user_id(), current_user_name();

  => 1004, EMB 

It is not clear what the meaning of this function is and how it is supposed to be used?