the following document [url]http://technet.oracle.com/docs/products/oracle8i/doc_library/817_doc/server.817/a85397/statem2a.htm#2056065
You can free only unused space above the high water mark (that is, the point beyond which database blocks have not yet been formatted to receive data).
I always had an impression that deallocated unused will free up a space below hight water mark.
According to the document located at the link above 'deallocate unused' does not re-gain anything below the hight water mark...
Please help me to understand this issue...
The high water mark is the upper limit on the data you have put in the table. When you delete data, the highwater mark does not get adjusted back down. Once these blocks have data in them they are no longer UNUSED. When you delete the data, the blocks may be empty, but they are not UNUSED.
There are only two ways to modify the highwater mark, TRUNCATE TABLE and MOVE TABLE.