A. Regular reduction of asset value to correspond to changes in market value as the asset ages
B. A process of correlating the market value of an asset with its gradual decline in physical efficiency
C. Allocation of cost in a manner that will ensure that Plant and Equipment items are not carried on the Balance Sheet in excess of net realizable value
D. Allocation of the cost of an asset to the periods in which services are received from the asset (Correct)

AS-6 defines depreciation as a measure of the wearing out, consumption or other loss of value of a depreciable asset arising from use, effluxion of time or obsolescence through technology and market changes. Depreciation is allocated so as to change a fair proportion of the depreciable amount in each accounting period during the expected useful life of the asset. Depreciation includes amortization of assets whose useful life is predetermined. The ultimate outcome of accounting for depreciation is cash available to replace the asset; however, this cannot be the purpose of depreciation.