You're seeing this on an 8i (or lower) database?

Oracle9i Release 2 - Managing Space in Tablespaces
Tablespaces allocate space in extents. Tablespaces can use two different methods to keep track of their free and used space:

Locally managed tablespaces: Extent management by the tablespace
Dictionary managed tablespaces: Extent management by the data dictionary
When you create a tablespace, you choose one of these methods of space management. You cannot alter the method at a later time.