Here is a cut and paste on this from "Expert One on One Oracle"

<quote>
Changes made to this view are immediately visible to other sessions - without the need to commit your transaction.  For any process that updates this view, you will be able to monitor their progress from another session by querying the V$SESSION_LONGOPS view. You too have the ability to populate rows in this view - typically 1 row but you may use others if you like.

The API to set the values in this view is defined as:

PROCEDURE SET_SESSION_LONGOPS
 Argument Name                  Type                    In/Out Default?
 ------------------------------ ----------------------- ------ --------
 RINDEX                         BINARY_INTEGER          IN/OUT
 SLNO                           BINARY_INTEGER          IN/OUT
 OP_NAME                        VARCHAR2                IN     DEFAULT
 TARGET                         BINARY_INTEGER          IN     DEFAULT
 CONTEXT                        BINARY_INTEGER          IN     DEFAULT
 SOFAR                          NUMBER                  IN     DEFAULT
 TOTALWORK                      NUMBER                  IN     DEFAULT
 TARGET_DESC                    VARCHAR2                IN     DEFAULT
 UNITS                          VARCHAR2                IN     DEFAULT 

Where:

o RINDEX: tells the server which row to modify in the V$SESSION_LONGOPS view. If you set this value to dbms_application_info.set_session_longops_nohint, a new row will be allocated in this view for you and the index of that row will be returned in RINDEX. Subsequent calls to SET_SESSION_LONGOPS with the same value for rindex will update that already existing row.

o SLNO: is an internal value. You should initially pass a null number in and ignore its value otherwise. You should pass the same value in with each call.

o OP_NAME: the name of the long running process.  It is limited to 64 bytes in size and should be set to some string that will be easily identified and provides some meaning to you.

o TARGET: Typically used to hold the object id that is the target of the long running operation (eg: the object id of the table being loaded). You may supply any number you wish here or leave it null.

o CONTEXT: A user defined number. This number would have meaning only to you - it is simply any number you wish to store.

o SOFAR: This is defined as any number you want to store but - if you make this number be some percentage or indicator of the amount of work done, the database will attempt to estimate your time to completion for you. For example, if you have 25 things to do and they all take more or less the same amount of time - you could set SOFAR to the number of things done so far and then set the next parameter TOTALWORK. The server will figure out how long it took you to get to where you are and estimate how long it will take you to complete.

o TOTALWORK: This is defined as any number you want to store but - the same caveat for SOFAR applies here. If SOFAR as a percentage of TOTALWORK represents your progress, the server will compute the time remaining to complete your task.

o TARGET_DESC: This is used to describe the contents of the TARGET input from above.  If the TARGET actually contained an object id, this might contain the object name for that object id.

o UNITS: A descriptive term that categorizes what SOFAR and TOTALWORK are measured in - units might be 'files' or 'iterations' or 'calls' for example.

These are the values you can set - when you look at the V$SESSION_LONGOPS view, you'll see it has many more columns than these however:

ops$tkyte@ORA8I.WORLD> desc v$session_longops
 Name                             Null?    Type
 -------------------------------- -------- ----------------------
 SID                                       NUMBER
 SERIAL#                                   NUMBER
 OPNAME                                    VARCHAR2(64) **
 TARGET                                    VARCHAR2(64) **
 TARGET_DESC                               VARCHAR2(32) **
 SOFAR                                     NUMBER **
 TOTALWORK                                 NUMBER **
 UNITS                                     VARCHAR2(32) **
 START_TIME                                DATE
 LAST_UPDATE_TIME                          DATE
 TIME_REMAINING                            NUMBER
 ELAPSED_SECONDS                           NUMBER
 CONTEXT                                   NUMBER **
 MESSAGE                                   VARCHAR2(512)
 USERNAME                                  VARCHAR2(30)
 SQL_ADDRESS                               RAW(4)
 SQL_HASH_VALUE                            NUMBER
 QCSID                                     NUMBER

The SID and SERIAL# columns are used to join back to V$SESSION to pick up the session information. The columns marked with ** are the ones you have control over and set. The START_TIME column marks the time this record was created (typically your first call to DBMS_APPLICATION_INFO.set_session_longops). The LAST_UPDATE_TIME column represents the time of your last call to set_session_longops. The TIME_REMAINING is an estimate in seconds of the time to completion, it is equal to round(elapsed_seconds*((totalwork/sofar)-1)). The ELAPSED_SECONDS column is the time in seconds since the start of the long running operation and the last update time. The MESSAGE column is a derived column - it concatenates together pieces of the OPNAME, TARGET_DESC, TARGET, SOFAR, TOTALWORK, and UNITS column to make a readable description of the work in process. The USERNAME is the name of the user this process is executing under. The SQL_ADDRESS and SQL_HASH_VALUE may be used to look into V$SQLAREA to see what SQL statement this process was last executing. The QCSID is used with parallel query - it would be the session of the parallel coordinator.
</quote>


So, these values are set by you -- the others are calculated as described.

In the event ORACLE is the one setting the values -- it depends on the process.  a full scan knows how many blocks it has to scan, how many it has scanned and so on -- so that is its units and so on. 

scott%ORA10GR2> select /*+ ordered index( dept, pk_dept) use_nl(emp dept) */ * from emp, dept where emp.deptno = dept.deptno;

Execution Plan
----------------------------------------------------------
Plan hash value: 351108634

------------------------------------------------------------------------
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%
------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |         |    14 |   798 |    17
|   1 |  NESTED LOOPS                |         |    14 |   798 |    17
|   2 |   TABLE ACCESS FULL          | EMP     |    14 |   518 |     3
|   3 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     1 |    20 |     1
|*  4 |    INDEX UNIQUE SCAN         | PK_DEPT |     1 |       |     0
------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   4 - access("EMP"."DEPTNO"="DEPT"."DEPTNO")

for x in ( select * from emp )
loop
    for y in (select * from dept where deptno = X.DEPTNO )
    loop
       output record
    end loop
end loop

ops$tkyte%ORA11GR2> declare
  2      l_nohint number default dbms_application_info.set_session_longops_nohint;
  3      l_rindex number default l_nohint;
  4      l_slno   number;
  5  begin
  6      for i in 1 .. 10
  7      loop
  8          dbms_lock.sleep(1);
  9          dbms_application_info.set_session_longops
 10          ( rindex =>  l_rindex,
 11            slno   =>  l_slno,
 12            op_name => 'my long running operation',
 13            target  =>  1234,
 14            target_desc => '"msg description"',
 15            context     => 0,
 16            sofar       => i,
 17            totalwork   => 10,
 18            units       => 'loops'
 19          );
 20      end loop;
 21  end;
 22  /