The Sixth Order was a military unit of the Cardassian Union. In 2371, Gul Evek commanded the Sixth Order from his ship, the Vetar. That year, he ordered the Sixth Order to Nramia to evacuate the planet after a devastating Maquis attack. (VOY - The Brave and the Bold, Book Two novella: The Third Artifact)
The unpublished material within the Ash & Iron: The Cardassians  were the Cardassian militaries deep space exploration division. During the Dominion War, its duties shifted to combat the Federation but suffered fewer losses compared to some of the other Orders..