Article 17 was a Gallifreyan law from the Constitution of the Time Lords. It was a "guarantee of liberty"; part of it affirmed that no candidate for presidency could in any way be debarred or restrained from presenting his claim.
The guarantee of liberty did not extend to condemned murderers.
The Fourth Doctor invoked it in order to suspend his own trial for the murder of the Lord President and delay the sentence. Goth intended to amend the article after his supposed election. (TV: The Deadly Assassin)