x$ktfbue:kernel transaction, file bitmap used extent,used extent bitmap in file header for LMT (equivalent to uet$ in DMT); check dba_extents view definition,ktfb –space/spcmgmt support for bitmapped space manipulation of files/tablespaces KTFBUE means K[Kernel] T[Transaction] F[File] B[Bitmap] U[Used] E[Extents]

SQL> desc x$ktfbue;

Name	        Meaning
ADDR	        N/A
INDX	        N/A
INST_ID	        N/A
KTFBUESEGTSN	ts# containing this segment
KTFBUESEGFNO	Relative number of the file containing the segment header
KTFBUESEGBNO	segment header block number
KTFBUEEXTNO	Extent number
KTFBUEFNO	Relative number of the file containing the extent
KTFBUEBNO	Starting block number of the extent
KTFBUEBLKS	Size of the extent in ORACLE blocks
KTFBUECTM	commit_jtime,Commit Time of the undo in the extent expressed as Julian date
KTFBUESTT	commit_wtime,Commit Time of the undo in the extent expressed as wall clock time
KTFBUESTA	Transaction Status of the undo in the extent;1, 'ACTIVE', 2, 'EXPIRED', 3, 'UNEXPIRED', 0 for non-undo-extent