Session Management - Oracle 10g/11g Data and Database Management Utilities - page 165

POST_TRANSACTION : This option leaves the user session alive, it allows the user
to complete any transaction activity. As soon as the user issues either a commit or
roll back command the session is disconnected.
IMMEDIATE : This simply kills the session. It releases all allocated session
resources, sends the ORA-00028 error message to the user, and kills the Oracle
server process. The DISCONNECT IMMEDIATE option behaves the same as the
KILL IMMEDIATE option.
In the above image it can be seen there is a blocking session ( A ) whose SID is 159 ,
according to V$SESSION this is blocking three other sessions, 138 , 153 , and 130 .
The DBA issues a simple KILL SESSION command ( B ), this kills the user session,
but as there are no parameters specified to kill the session, the default behavior
of this command will simply kill the session, the entry at the V$SESSION will
remain there until somehow the oracle server detects some user activity so it can
send the ORA-00028 error message. When monitoring the sessions, the KILLED
status ( C ), as well as the new blocking session, session 138 ( D ), which was the next
session in line on the enqueue mechanism, all killed sessions are reported in the
alert <SID>.log file.