SQL> describe PURCHASEORDER
Name                    Null?           Type
--------------------	-------------	---------------

TABLE of SYS.XMLTYPE(XMLSchema "http://asktom.oracle.com/~sdillon/xsd/purchaseOrder.xsd" Element "PurchaseOrder") STORAGE Object-relational TYPE "XDBPO_TYPE"

The PURCHASEORDER table is a table of XMLType, meaning its sole purpose is to store XML data. The description shows that the XMLType table is based on an XML Schema and uses an object-relational storage architecture based on the SQL object type XDBPO_TYPE. If I go on to describe XDBPO_TYPE, I see the SQL object type that was generated to store XML data for this XML Schema:

SQL> describe XDBPO_TYPE
XDBPO_TYPE is NOT FINAL
Name                        Null?                       Type

----------------------      --------------------       ------------------
SYS_XDBPD$                                  
XDB.XDB$RAW_LIST_T
Reference                                   		VARCHAR2(30)
ACTIONS                                     
XDBPO_ACTIONS_TYPE
REJECTION                                  

XDBPO_REJECTION_TYPE
Requestor                                   		VARCHAR2(128)
User                                        		VARCHAR2(10)
CostCenter                                  		VARCHAR2(4)
SHIPPINGINSTRUCTIONS                       
XDBPO_SHIPINSTRUCTIONS_TYPE
SpecialInstructions                         		VARCHAR2(2048)
LINEITEMS                                   

XDBPO_LINEITEMS_TYPE