Query on Volume pool

First, for Netbackup pool, it is defined as default pool incase of volume pool not specificed in the policy. But as my understanding, at the time of policy creation, netbackup will be selected by default and this can be changed to user defined if required. Nothing else than this, if I am right.

the above is correct

Secondly, could you please descript where None and Datastore volume pool will come into picture.

The None pool is for cleaning media.  The Datastore pool is for a Datastore policy type.  A Datastore policy type is used with the SDK so you can create your own automated backup feature in an application.  For example Enterprise vault may use the Datastore policy for migrations to NetBackup.

Thirdly, If we expire all the images in the media will move the tape to scratch pool or is that will remains in the same pool. When I tried, it resides in the same pool, but there are a definitions in web that is says, once all the images are expired automatically or by force will move the tape to scratch pool.

Tapes when expired goes back to where it came from.  So if the tape was in scratch before the backup, and then moved to another volume when the backup grabbed it.  It will go back to scratch when the tape is expired.  However if you manually moved the tape to another pool, let's say NetBackup.  And then NetBackup perform a backup to that tape in NetBackup.  When it expires, it will stay in NetBackup.