(a) Gains and losses on retirements. For the purposes of this section the term "retirement" means the permanent withdrawal of depreciable property from use in the trade or business or in the production of income. The withdrawal may be made in one of several ways. For example, the withdrawal may be made by selling or exchanging the asset, or by actual abandonment. In addition, the asset may be withdrawn from such productive use without disposition as, for example, by being placed in a supplies or scrap account. The tax consequences of a retirement depend upon the form of thetransaction, the reason therefor, the timing of the retirement, the estimated useful life used in computing depreciation, and whether the asset is accounted for in a separate or multiple asset account. Upon the retirement of assets, the rules in this section apply in determining whether gain or loss will be recognized, the amount of such gain or loss, and the basis for determining gain or loss: