When an Oracle table is created with an XMLTYPE column but without any XML schema specification, a hidden CLOB column is automatically created to store the XML data. The XMLTYPE column becomes a virtual column for the hidden CLOB column. In the corresponding Oracle base table, the hidden column immediately follows the XMLTYPE column that it represents and is named SYS_NCnnnnn$, where nnnnn represents the position of the hidden column in the base table.
For example, a table is created in the Oracle database with this DDL command:
CREATE TABLE sampletable ( col1 INT, , col2 INT, , xml1 XMLTYPE , xml2 XMLTYPE);
The Oracle database creates hidden columns named SYS_NC00004$ and SYS_NC00006$, which respectively correspond to the xml1 and xml2 columns. These hidden CLOB columns cannot be accessed directly. However, they can be viewed by querying the col$ and obj$ base tables of the Oracle data dictionary. See the Oracle documentation.