Tex. Utilities Code § 39.260

(a) The definition and identification of invested capital and other terms used in this subchapter and Subchapter G that affect the net book value of generation assets and the treatment of transactions performed under Section 35.035 and other transactions authorized by this title or approved by the regulatory authority that affect the net book value of generation assets during the freeze period shall be treated in accordance with generally accepted accounting principles as modified by regulatory accounting rules generally applicable to utilities.

(b) The principles and criteria described by Subsection (a), including the criteria for applicability of Statement of Financial Accounting Standards No. 71 ("Accounting for the Effects of Certain Types of Regulation"), shall be applied for purposes of this subchapter as they existed on January 1, 1999.