Why you should use Automatic segment space management (ASSM)

Automatic segment space management (ASSM) is a simpler and more efficient way of managing space within a segment. It completely eliminates any need to specify and tune the pctused, freelists, and freelist groups storage parameters for schema objects created in the tablespace. If any of these attributes are specified, they are ignored.  ASSM is not for every database, especially those with super-high DML rates:

Varying row sizes: ASSM is better than a static pctused. The bitmaps make ASSM tablespaces better at handling rows with wide variations in row length.

Reducing buffer busy waits: ASSM will remove buffer busy waits better than using multiple freelists. When a table has multiple freelists, all purges must be parallelized to reload the freelists evenly, and ASSM has no such limitation.

Great for Real Application Clusters: The bitmap freelists remove the need to define multiple freelists groups for RAC and provide overall improved freelist management over traditional freelists.