CREATE TABLE t
    (x_customer_po                  VARCHAR2(20 BYTE),
    x_accessory_code               VARCHAR2(3 BYTE),
    x_install_loc                  CHAR(3 BYTE));

INSERT INTO apps.T 
VALUES('N45882','D0C','MFG');
INSERT INTO apps.T 
VALUES('N45882','K0B','MFG');
INSERT INTO apps.T 
VALUES('N45882','LK2','MFG');
INSERT INTO apps.T 
VALUES('N45882','R0B','MFG');
INSERT INTO apps.T 
VALUES('N45883','B0A','MFG');
INSERT INTO apps.T 
VALUES('N45883','M0E','MFG');
INSERT INTO apps.T 
VALUES('N45883','R0B','MFG');
INSERT INTO apps.T 
VALUES('N45884','D0A','MFG');
INSERT INTO apps.T 
VALUES('N45884','K0B','MFG');
INSERT INTO apps.T 
VALUES('N45884','LK2','MFG');
INSERT INTO apps.T 
VALUES('N45884','Q0F','MFG');
INSERT INTO apps.T 
VALUES('N45885','D00','MFG');
INSERT INTO apps.T 
VALUES('N45885','H8B','MFG');
INSERT INTO apps.T 
VALUES('N45886','D0A','MFG');
INSERT INTO apps.T 
VALUES('N45886','D0B','MFG');
INSERT INTO apps.T 
VALUES('N45886','OYF','MFG');
INSERT INTO apps.T 
VALUES('N45887',NULL,'MFG');
INSERT INTO apps.T 
VALUES('N45888',NULL,'MFG');
COMMIT;

select XMLSerialize
    ( DOCUMENT(  
 SELECT XMLAGG ( 
       XMLELEMENT ( "OrderAcknowledgments",
           XMLELEMENT ("Sender", 
                  XMLATTRIBUTES (decode (fnd_global.CONC_REQUEST_ID,-1,999999999) AS "ID",
                                 'Cars' AS "Name"  )
                       ), --Sender      
           XMLELEMENT ( "DateTime",REPLACE (to_char(systimestamp,'YYYY-MM-DD HH24:MI:SS.FF1'),' ','T') ),
           XMLELEMENT ("Acknowledgments",
                (SELECT xmlagg( 
                        XMLELEMENT ("Acknowledgment",
                            XMLELEMENT ( "Vehicle",
                                XMLELEMENT ("CustomerPONum",   x_customer_po) ,                            
                                XMLELEMENT ("Options",
                                            XMLAGG (
                                                XMLELEMENT ("Option",                                                    
                                                        XMLForest ( x_accessory_code AS "Code", 
                                                                    x_install_loc AS  "InstallLocation")
                                                   
                                                            ) --"Option"
                                                        )
                                             -- WHERE t.X_CUSTOMER_PO = x_customer_po 
                                            --GROUP BY x_customer_po
                                             --xmlag
                                           ) --options
                                       )--vehicles
                                       
                                     ) --ack
                                 )--xmlagg
                     FROM t
                     GROUP BY x_customer_po
                     ) ----select
                       ) --acks
               ) --orderacks
           )--xmlagg
    FROM dual ) --select
     )--document
FROM dual;

-- default no indent:
SCOTT@orcl_11gR2> select xmlserialize
  2  	      (document xmltype
  3  		 ('<a><b><c>something</c></b></a>'))
  4  from   dual
  5  /

XMLSERIALIZE(DOCUMENTXMLTYPE('<A><B><C>SOMETHING</C></B></A>'))
--------------------------------------------------------------------------------
<a><b><c>something</c></b></a>

1 row selected.

-- indent with default size of 2 spaces:
SCOTT@orcl_11gR2> select xmlserialize
  2  	      (document xmltype
  3  		 ('<a><b><c>something</c></b></a>')
  4  	       indent)
  5  from   dual
  6  /

XMLSERIALIZE(DOCUMENTXMLTYPE('<A><B><C>SOMETHING</C></B></A>')INDENT)
--------------------------------------------------------------------------------
<a>
  <b>
    <c>something</c>
  </b>
</a>


1 row selected.

-- indent 5 spaces:
SCOTT@orcl_11gR2> select xmlserialize
  2  	      (document xmltype
  3  		 ('<a><b><c>something</c></b></a>')
  4  	       indent size = 5)
  5  from   dual
  6  /

XMLSERIALIZE(DOCUMENTXMLTYPE('<A><B><C>SOMETHING</C></B></A>')INDENTSIZE=5)
--------------------------------------------------------------------------------
<a>
     <b>
          <c>something</c>
     </b>
</a>


1 row selected.

-- should not be indented, but is:
SCOTT@orcl_11gR2> select xmltype ('<a><b><c>something</c></b></a>')
  2  from   dual
  3  /

XMLTYPE('<A><B><C>SOMETHING</C></B></A>')
--------------------------------------------------------------------------------
<a>
  <b>
    <c>something</c>
  </b>
</a>


1 row selected.

-- should produce indentation but does the opposite:
SCOTT@orcl_11gR2> select xmltype ('<a><b><c>something</c></b></a>').extract('*')
  2  from   dual
  3  /

XMLTYPE('<A><B><C>SOMETHING</C></B></A>').EXTRACT('*')
--------------------------------------------------------------------------------
<a><b><c>something</c></b></a>

1 row selected.

SQL> select xmltype ('<a><b><c>something</c></b></a>')
  2  from   dual
  3  /
XMLTYPE('<A><B><C>SOMETHING</C></B></A>')
-------------------------------------------------------------------------
<a><b><c>something</c></b></a>

1 row selected.

SQL> select xmltype ('<a><b><c>something</c></b></a>').extract('*')
  2  from   dual
  3  /
XMLTYPE('<A><B><C>SOMETHING</C></B></A>').EXTRACT('*')
-------------------------------------------------------------------------
<a><b><c>something</c></b></a>

1 row selected.