On a party's motion or on its own initiative, a trial court may permit an appeal from an interlocutory order that is not otherwise appealable, as provided by statute. Permission must be stated in the order to be appealed. An order previously issued may be amended to include such permission. The permission must identify the controlling question of law as to which there is a substantial ground for difference of opinion, and must state why an immediate appeal may materially advance the ultimate termination of the litigation.
Tex. R. Civ. P. 168