Oracle Database Startup Restricted Feature.

STARTUP RESTRICTED 

when to use

1. perform import or export of data
2. Data load using sql*loader
3. temporarily prevent users from accessing the data.
4. Migration or upgrade operations.


Only users with with CREATE SESSION and RESTRICTED SESSION
can access a database opened in restricted mode.

Also, the database administrator cannot access the instance 
remotely through Oracle Net Listener, But can do so only 
through local access to the machine.

SQL> startup restrict

ORACLE instance started.

Total System Global Area  619360256 bytes
Fixed Size                  1338280 bytes
Variable Size             398459992 bytes
Database Buffers          213909504 bytes
Redo Buffers                5652480 bytes
Database mounted.
Database opened.

SQL> select logins from v$instance;

LOGINS
----------
RESTRICTED


SQL> alter system disable restricted session;

System altered.

SQL> select logins from v$instance;

LOGINS
----------
ALLOWED


SQL> alter system enable restricted session;

System altered.

SQL> select logins from v$instance;

LOGINS
----------
RESTRICTED