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