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
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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