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	        MeaningADDR	        N/AINDX	        N/AINST_ID	        N/AKTFBUESEGTSN	ts# containing this segmentKTFBUESEGFNO	Relative number of the file containing the segment headerKTFBUESEGBNO	segment header block numberKTFBUEEXTNO	Extent numberKTFBUEFNO	Relative number of the file containing the extentKTFBUEBNO	Starting block number of the extentKTFBUEBLKS	Size of the extent in ORACLE blocksKTFBUECTM	commit_jtime,Commit Time of the undo in the extent expressed as Julian dateKTFBUESTT	commit_wtime,Commit Time of the undo in the extent expressed as wall clock timeKTFBUESTA	Transaction Status of the undo in the extent;1, 'ACTIVE', 2, 'EXPIRED', 3, 'UNEXPIRED', 0 for non-undo-extent