what is the use of data classification in SLP?

Verbatim from KBs -

A data classification represents a set of backup requirements, which makes it easier to configure backups for data with different requirements. For example, email data and financial data...

The Data Classification attribute specifies the classification of the storage lifecycle policy that stores the backup. For example, a backup with a gold classification must go to a storage unit with a gold data classification. By default, NetBackup provides four data classifications: platinum, gold, silver, and bronze.

This attribute is optional and applies only when the backup is to be written to a storage lifecycle policy. If the list displays No data classification, the policy uses the storage selection that is displayed in the Policy storage list. If a data classification is selected, all the images that the policy creates are tagged with the classification ID.

so what is the use of that id?

OK, think of it like this, it is a way to set how important your data is.

For example, Gold data is more valuable / important to your business than bronze data.

To be honest, I don;t see tahth many people using it, yes, some do, but given all the SLP cases I see, very few use data classification.

Martin

ok .so how nbu treats this data.silver data will be duplicated after gold?is it like this?

No, it has no effect on when it is duplicated.

"A data classification is assigned in the backup policy to associate backup data with a data classification. Data from the policy can be stored only in a storage lifecycle policy with the same data classification assigned to it"

Martin

if we dont assign data classification then?

Once set, data assigned as gold can only be sent to a ''gold' storage unit.

and 

A data classification is assigned in the backup policy to associate backup data with a data classification. Data from the policy can be stored only in a storage lifecycle policy with the same data classification assigned to it.

Whats the difference between this?

Why dont you put some time to read documents... ?