TexasCHLforum.com

My understanding, which may be defective, is: A defense to prosecution means that you can use it as a defense when you are prosecuted, with the court's permission, and you have to prove you were doing that which is "a defense to prosecution."

Thus:

§ 46.03. PLACES WEAPONS PROHIBITED. (a) A person commits
an offense if the person intentionally, knowingly, or recklessly
possesses or goes with a firearm, illegal knife, club, or
prohibited weapon listed in Section 46.05(a):
(1) on the physical premises of a school or
educational institution, any grounds or building on which an
activity sponsored by a school or educational institution is being
conducted, or a passenger transportation vehicle of a school or
educational institution, whether the school or educational
institution is public or private, unless pursuant to written
regulations or written authorization of the institution;
(2) on the premises of a polling place on the day of an
election or while early voting is in progress;
(3) on the premises of any government court or offices
utilized by the court, unless pursuant to written regulations or
written authorization of the court;
(4) on the premises of a racetrack;
(5) in or into a secured area of an airport; or
(6) within 1,000 feet of premises the location of
which is designated by the Texas Department of Criminal Justice as a
place of execution under Article 43.19, Code of Criminal Procedure,
on a day that a sentence of death is set to be imposed on the
designated premises and the person received notice that:
(A) going within 1,000 feet of the premises with
a weapon listed under this subsection was prohibited; or
(B) possessing a weapon listed under this
subsection within 1,000 feet of the premises was prohibited.
(b) It is a defense to prosecution under Subsections
(a)(1)-(4) that the actor possessed a firearm while in the actual
discharge of his official duties as a member of the armed forces or
national guard or a guard employed by a penal institution, or an
officer of the court.

Means that a person who can prove he possessed a firearm while in actual discharge, etc, etc, etc, then he can present that as a defense.

I think?