When a table space is created, EXTENTSIZE n can be optionally specified, where n is the extent size. If you do not specify the extent size on the CREATE TABLESPACE statement, the database manager uses the value given by this parameter.
Recommendation: In many cases, you will want to explicitly specify the extent size when you create the table space. Before choosing a value for this parameter, you should understand how you would explicitly choose an extent size for the CREATE TABLESPACE statement.