Storage_Clause:

INITIAL

Specify in bytes the size of the object’s first extent. Oracle allocates space for this extent when you create the schema object. Use K or M to specify this size in kilobytes or megabytes.

The default value is the size of 5 data blocks. In tablespaces with manual segment-space management, the minimum value is the size of 2 data blocks plus one data block for each free list group you specify. In tablespaces with automatic segment-space management, the minimum value is 5 data blocks. The maximum value depends on your operating system.

In dictionary-managed tablespaces, if MINIMUM EXTENT was specified for the tablespace when it was created, then Oracle rounds the value of INITIAL up to the specified MINIMUM EXTENT size if necessary. If MINIMUM EXTENT was not specified, then Oracle rounds the INITIAL extent size for segments created in that tablespace up to the minimum value (see preceding paragraph), or to multiples of 5 blocks if the requested size is greater than 5 blocks.

In locally managed tablespaces, Oracle uses the value of INITIAL in conjunction with the size of extents specified for the tablespace to determine the object’s first extent. For example, in a uniform locally managed tablespace with 5M extents, if you specify an INITIAL value of 1M, then Oracle creates five 1M extents.

Restriction on INITIAL: You cannot specify INITIAL in an ALTER statement.

MINEXTENTS

Specify the total number of extents to allocate when the object is created. This parameter lets you allocate a large amount of space when you create an object, even if the space available is not contiguous. The default and minimum value is 1, meaning that Oracle allocates only the initial extent, except for rollback segments, for which the default and minimum value is 2. The maximum value depends on your operating system.

If the MINEXTENTS value is greater than 1, then Oracle calculates the size of subsequent extents based on the values of the INITIAL, NEXT, and PCTINCREASE storage parameters.

When changing the value of MINEXTENTS (that is, in an ALTER statement), you can reduce the value from its current value, but you cannot increase it. Resetting MINEXTENTS to a smaller value might be useful, for example, before a TRUNCATE … DROP STORAGE statement, if you want to ensure that the segment will maintain a minimum number of extents after the TRUNCATE operation.
Restriction on MINEXTENTS

You cannot change the value of MINEXTENTS for an object that resides in a locally managed tablespace.

MAXEXTENTS

Specify the total number of extents, including the first, that Oracle can allocate for the object. The minimum value is 1 (except for rollback segments, which always have a minimum value of 2). The default value depends on your data block size.
Restriction on MAXEXTENTS

You cannot change the value of MAXEXTENTS for an object that resides in a locally managed tablespace.

UNLIMITED

Specify UNLIMITED if you want extents to be allocated automatically as needed. Oracle Corporation recommends this setting as a way to minimize fragmentation.

However, do not use this clause for rollback segments. Rogue transactions containing inserts, updates, or deletes that continue for a long time will continue to create new extents until a disk is full.

Reference:

1. http://www.cs.bris.ac.uk/maintain/oracle9docs/server.920/a96540/clauses9a.htm