Although it is difficult to tell, the Assault Frigate Mark I is actually a highly modified Imperial Dreadnaught heavy cruiser.
To economize fuel consumption and increase capacity, the techs stripped off huge amounts of the superstructure. They also added three massive dorsal fins to increase the ship's maneuverability.
In addition, huge portions of the ship have been retooled to replace human crew with automation. The original Imperial Dreadnaught required a crew of 16,000 in order to operate, far more than any other war vessel of comparable size. This is simply not practical for the Alliance, who has a much smaller manpower pool to draw from than does the Navy, and an infinitely smaller pool of trained crew. Once the Alliance techs were through with the ship, it required a little less than 1/8 the original complement to operate. Unfortunately, the Assault Frigate Mk. I did have to sacrifice offensive capability -- while it has additional weaponry, crew members and power generators were diverted from the interior of the ships, resulting in a lower fire rate for the turbolasers compared to the Dreadnaught.
Unlike its Imperial predecessor, the Assault Frigate Mark I has no docking bays or landing platforms, but there are 20 umbilical docking fixtures along the ship's surface. Several of these are large enough to dock a transport/heavy freighter, while the rest are better suited to starfighters and light freighters. The ship cannot carry docked craft through hyperspace.
In addition, each assault frigate carries a modified Imperial assault shuttle piggy-back atop its superstructure. The frigates are able to carry these vessels through hyperspace.
Because of the exhaustive and expensive refit, there are very few assault frigates currently employed by the Alliance. Those that are in services are extremely effective, however, and, as time permits, more will be added.