Keep on reading....

Read this topic in Admin Guide I: Reduce fragment size storage unit setting.

Read the explanation in Table 11-5.

Do a test backup with default fragment size directly to tape. It must be a large backup with lots of files (File server folders will be ideal.)

After you have read this, go apply on your Storage Units. (Some people prefer a value of 2000, but we also see good results with 5000.)
Rerun the same test backup.

Then perform a test restore (one file to a temp folder) from the backup that was done before the change.

Then perform a test restore from the backup after the change.

Please tell us the result.

PS: Do you know Arun K and NIKHIL2346565959?

No.I dont know them.

I have done the restore.One is fast and one is slow.

So on tape we have tape marks which is called fragment size?

Excellent! You have figured it out!

This is how all of us learn.
Sometimes we read something in the manual that we don't fully understand.
We then go an test it on live or test environment. 
When we see the results, we understand the theory in the manual.

The type of questions you ask are similar to those of NIKHIL and Arun.....

Thanks.But i waana know what exactly a tape has?

It has tar files?