SQL> select SYS_CONTEXT('USERENV','CURRENT_USER') from dual
  2  ;

SYS_CONTEXT('USERENV','CURRENT_USER')
--------------------------------------------------------------
OWB_TARGET

SQL> select SYS_CONTEXT('USERENV','MODULE') from dual ;
select SYS_CONTEXT('USERENV','MODULE') from dual
                                            *
ERROR at line 1:
ORA-02003: invalid USERENV parameter


SQL> 

SQL>  select SYS_CONTEXT('USERENV','MODULE') from dual ;
SYS_CONTEXT('USERENV','MODULE')
----------------------------------------------------------
SQL*Plus

1 row selected.

SQL> l
  1  CREATE OR REPLACE TRIGGER BOI_login_restrict AFTER LOGON ON DATABASE
  2  DECLARE
  3          mod_out VARCHAR2(48);
  4     act_out VARCHAR2(32);
  5  BEGIN
  6    dbms_application_info.read_module(mod_out, act_out);
  7    IF (SYS_CONTEXT('USERENV','CURRENT_SCHEMA') in ('OWB_TARGET','BOIDW')
  8      and mod_out in ('SQL*Plus','T.O.A.D.')
  9           and SYS_CONTEXT('USERENV','OS_USER') = 'nazaretr') THEN
 10          RAISE_APPLICATION_ERROR(-20003,'You are not allowed to connect to the database with this Schema!!!');
 11    END IF;
 12* END;
SQL>