Depending on the cost flow assumption specified for inventory, inventory is modified at period end to the lower of cost or net realizable value or to the lower of cost or market.

At the conclusion of an accounting period, accounts receivable are modified to net realizable value.

In some circumstances, the fair value of long-term operating assets is increased after an asset impairment assessment.

Any gains in value for any of these items are prohibited until realization

1- What concept or principle underlies the valuation of these items ?