The Confirm Reallocate Space dialog is only for a for-load database. It is used only after creating a database for load, when you must re-create the space allocations and segment usage of the database being loaded from a dump.
As a database grows, you may need to increase the storage space allocated to it. After allocating storage space, you cannot de-allocate it, you can only add to it, or re-allocate it.
Right click a for-load database and select “Reallocate Space” menu item to bring up the dialog.
Only a system administrator can change storage allocations.
To reduce
the size of a database, you must back up all the data, delete the database,
recreate it with smaller storage allocations, and then reload the
data.