The Cardassian Eleventh Order was a Cardassian military division.
The Eleventh Order took on the Klingon Empire during the Dominion War when the Klingons attacked Septimus III. Damar asked Weyoun for Jem'Hadar reinforcements, but Weyoun decided against sending any, having decided Septimus III was not a strategic planet. Five hundred Cardassian soldiers were killed in the fighting, the entire Eleventh Order. (DS9 episode: "Strange Bedfellows")
The news of the loss of the Eleventh Order reached the troops at Keralek Base on Ogyas III soon after. (DS9 novel: The Never-Ending Sacrifice)