Why did "CSSCAN" skip the clob column and xmltype column ?

Hello I'm Yuji
Why did "CSSCAN" skip the clob column and xmltype column ?

My database version is 11.2.0.3.

I ran the "CSSCAN" against my tables (their structure are below)

# racdb012(AZUMA)> desc xmltesttab
#  Name                                                  Null?    Type
#  ----------------------------------------------------- -------- ------------------------------------
#  ID                                                             NUMBER
#  XMLCOL                                                         PUBLIC.XMLTYPE STORAGE BINARY
#
# racdb012(AZUMA)> desc temp
#  Name                                                  Null?    Type
#  ----------------------------------------------------- -------- ------------------------------------
#  ID                                                             NUMBER
#  XMLCOL                                                         CLOB
#  

This is terminal-log when I ran the "CSSCAN".
It seems "CSSCAN" skipped all columns.

# >CSSCAN azuma/xxxxx
#
# Character Set Scanner v2.2 : Release 11.2.0.3.0 - Production on Mon Dec 2 14:00:13 2013
# Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.
#
# Connected to:
# Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production