The Super-class Star Destroyer was a fictional Star Destroyer model invented for budgetary reasons, whose name later became a popular military colloquialism for heavy warships of the Galactic Empire and successor-governments.
The Super-class was described as 8,000 meters long, armed with 250 turbolaser batteries, 250 heavy turbolaser batteries, 250 ion cannons, 250 concussion missile tubes, and 40 tractor beam projectors.
It would carry twelve TIE squadrons (including two TIE Interceptor squadrons) and more than fifty other small craft. For ground operations, it would carry a full corps of ground troops, apparently mixing stormtrooper legions with regular Army personnel, plus 25 AT-ATs, 50 AT-STs, and three prefabricated garrison bases. It would have been crewed by 280,734.
A single ship of this class would be considered the equal of ten or twenty Imperial-class Star Destroyers, and they were meant to be used as command ships by many of the most important Sector Fleets in the Empire.
The fake schematics of the Executor.
The name "Super-class Star Destroyer" was invented by Imperial Navy officials and used in its budget requests to the Imperial Senate during the design and construction phase of the Executor-class Star Dreadnought. This ruse was made to hide the true nature of the class, even going so far as to understate its size and armament, so that oversight committees would not discover its true role.
By the time the first vessel, Executor, was operational, the official designation had been changed to its original intention, but Super Star Destroyer and Super-class Star Destroyer had stuck in people's minds and was applied to other models, like the Sovereign-class and Eclipse-class.
One of these budget requests which included the "Super-class" and several other capital ship designs was sent to Palpatine's advisors on budgetary affairs. This document found its way into the hands of the Rebel Alliance, where it was used as a primary source regarding the Empire's capital ship capabilities by Arhul Hextrophon.
The Empire publicly revealed that four Super-class Star Destroyers, specifically Executor-class vessels, were completed by the Battle of Hoth. Circa 1 BBY, Imperial gunner Tenn Graneet hoped he would be assigned to one of them.
Even into the time of the New Jedi Order, both the terms "Super Star Destroyer" and "Super-class Star Destroyer" were still in use, and over the decades came to refer to several distinct designs.
The Megador, a Star Dreadnought with sixteen ion engines, was also referred to as a Super-class Star Destroyer during the Battle of Uroro Station.
Since 2004, the Executor-class were described as Star Dreadnoughts at 19 kilometers in length. The issue had some controversy, as many older sources described the Executor as either an 8- or 12.8-kilometer-long Super-class Star Destroyer.
On March 9, 2006, the Wizards of the Coast Star Wars Q&A section reiterated that the Executor was 19 kilometers, "as are all Super Star Destroyers of the same class." This kept the 8-kilometer Super-class as a possibility, as such ships could belong to a different class from Executor. However, with the Starship Battles Preview 1 preview article on October 19, 2006, the Super-class was retconned as explicitly fake, being used to hide the much larger Executor Star Dreadnought class.
On November 16, 2007, the official Databank updated the Executor article, pointing out that the term "Super-class Star Destroyer" was only used unofficially, alongside "Super Star Destroyer," with "Executor-class Star Dreadnought" being the ship class's real designation. The updated edition of Starships of the Galaxy repeated the information, even adding more Super Star Destroyer variants to the mix and using both "Super Star Destroyer" and "Super-class Star Destroyer" interchangeably as general descriptions.