If a value is placed in the landed cost or sales cost field, then this % value of the Actual Cost will be added to the Actual Cost to generate the Landed Cost value.
For example, if the Actual cost is $10, and you put a landed cost % of 10%, then the actual cost will remain at $10, and the new landed cost will calculate as $11, to show the 10% increase.
A default % can be added to all Products via the Administration (Manage System Keys>Pricing System Keys), if you prefer all of your Products to have this lift on Cost Prices..