For purposes of this subchapter, any conviction not obtained from a prosecution under this code shall be classified as follows:
(1) "felony of the third degree" if imprisonment in the Texas Department of Criminal Justice or another penitentiary is affixed to the offense as a possible punishment;
(2) "Class B misdemeanor" if the offense is not a felony and confinement in a jail is affixed to the offense as a possible punishment;
(3) "Class C misdemeanor" if the offense is punishable by fine only.