Each ARO must have at least one layer, reflecting a single estimated settlement cost with a particular effective date and end (settlement) date. You may later need to increase or decrease the expected cost, beyond the ordinary change due to inflation. To do this, add another layer to an existing ARO. This is different from revising an existing layer, which allows you to change the rate for inflation or present valuing. A separate layer is required for a change to the estimated cost because if the cost is increased, the additional cost is to be present valued at the current risk-free rate; a reduction in cost is booked using the original risk-free rate.