DBA_LMT_USED_EXTENTS

DBA_LMT_USED_EXTENTS describes the extents comprising the segments in all locally managed tablespaces in the database.

Column	Datatype	NULL	Description
SEGMENT_FILEID	NUMBER		File number of the segment header of the extent
SEGMENT_BLOCK	NUMBER		Block number of the segment header of the extent
TABLESPACE_ID	NUMBER		Identifier number of the tablespace containing the extent
EXTENT_ID	NUMBER		Extent number in the segment
FILEID	NUMBER		File identifier number of the file containing the extent
BLOCK	NUMBER		Starting block number of the extent
LENGTH	NUMBER		Number of blocks in the extent

Oracle data dictionary views

Oracle dynamic performance views