Seventh was the rank given by the Dominion to a Jem'Hadar who was seventh in his unit.
After Taran'atar once spoke in a derogatory fashion about a Thepnossen dance in front of Eris, the Jem'Hadar Second in his unit had him reduced to seventh from sixth. (DS9 - Gateways novel: Demons of Air and Darkness)
Other Jem'Hadar sevenths included Yak'Talon, Rotan'talag and Vorgan'lorat. (DS9 episode: "One Little Ship", DS9 - Worlds of Star Trek: Deep Space Nine novel: The Dominion: Olympus Descending, ST - Typhon Pact novel: Raise the Dawn)