x$ktfbue:kernel transaction, file bitmap free extents,Free extent bitmap in file header for LMT (equivalent to fet$ in DMT); check dba_free_space view definition,ktfb –space/spcmgmt support for bitmapped space manipulation of files/tablespaces KTFBFE means K[Kernel] T[Transaction] F[File] B[Bitmap] F[Free] E[Extents] 
SQL>  desc x$ktfbfe;
 Name                                     
 ----------------------------------------- 
 ADDR               N/A                            
 INDX               N/A                      
 INST_ID            N/A                         
 KTFBFETSN          TS#  containing this free extent                       
 KTFBFEFNO          Relative number of the file containing the free extent                             
 KTFBFEBNO          Starting block number of the free extent                                
 KTFBFEBLKS         Size of the extent in ORACLE blocks