Locally Managed Tablespaces
When Oracle allocates space to a segment (like a table or index), a group of contiguous free blocks, called an extent, is added to the segment. Metadata regarding extent allocation and unallocated extents are either stored in the data dictionary, or in the tablespace itself. Tablespaces that record extent allocation in the dictionary, are called dictionary managed tablespaces, and tablespaces that record extent allocation in the tablespace header, are called locally managed tablespaces.
Dictionary Managed Tablespaces (DMT):
Oracle use the data dictionary (tables in the SYS schema) to track allocated and free extents for tablespaces that is in "dictionary managed" mode. Free space is recorded in the SYS.FET$ table, and used space in the SYS.UET$ table.
