Classify data in accordance with how critical it is to your business and its value in terms of replacement and the affects of its loss or being compromised to your business.
This will help you build an It disaster recovery planning regime that is easy to manage and implement in the event of a security breach, so you can return to normal in the shortest possible time, with the least amount of disruption.
Consult staff and business managers to determine which areas are vital for them to complete their roles and which areas are less critical. Some of your data may not affect your day to day business or efficiency, so can be deferred until more critical data is recovered or restored