Depreciation Area 20- Cost Depreciation

Hi,

Can any one explain for what purpose depreciation area 20 can be configured in Asset Accounting and how it works?

Also I want to create only one depreciation for Asset Accounting i.e. Cost Depreciation Area 20 and no book depreciation area.

Then will this arrangement work correctly.

My business requirement is:

To use Asset Accounting in integration with PM for starting the replacement planning process for  the Equipment 3 yrs in advance

and to calculate net book value of Asset at the time of starting replacement planning.

regards

Shailaja