Remaining built-in gain
(13) Remaining built-in gain(i) In general. Remaining built-in gain is, with respect to section 721(c) property subject to the gain deferral method, the built-in gain reduced by decreases in the difference between the property's book value and adjusted tax basis, but, for purposes of this paragraph (b)(13)(i), without taking into account increases or decreases to the property's book value pursuant to 1.7041(b)(2)(iv)(f) or (s).