The MC40 was a small Rebel cruiser, larger than the Imperial Strike-class medium cruiser. It was equipped with powerful shields for a vessel of its size. Its three engines gave it a higher speed than the bigger MC80a Star Cruiser, and it was more maneuverable. The MC40 had a large, portside hangar and could carry one starfighter squadron.
The MC40 light cruiser was constructed some time prior to the Battle of Hoth. It saw heavy use in the year leading up to the Battle of Endor.
Ships of this kind accompanied larger vessels like the MC80 Star Cruiser. During the New Jedi Order era the New Republic relocated most of it's fleet to Kuat. These ships consisted of many Mon Calamari cruisers including MC40s and was established to strike the Yuuzhan Vong.