Heavy tank
The Char 2C was one of the largest tanks ever produced. At the start of World War II, France and the Soviet Union were the only countries to have inventories of heavy tanks, such as the Char B1, T-35, and KV-1. The Matilda II was designed under the British infantry tank concept which is similar to the heavy tank having thick armor and weighing more than their other tanks. It’s usually separate because they have less firepower with their cruiser tanks (comparable to mediums) at the time having the same main armament but more machine guns. Later war examples were the German Tiger I and II, as well as the Soviet IS series. Note that "heavy" versus "medium" is more a question of tactical roles than weight; the Panther, for example, was a "medium" tank that outweighed most Allied "heavy" tanks.
Tanks in the British Army
In 1939, most tanks had maximum armour of 30 mm or less, with guns no heavier than 37–47 mm. Medium tanks of 1939 weighed around 20 tons. Also if the tank's gun was to be used to engage both unarmoured and armoured targets, then it needed to be as large and powerful as possible, making one large gun with an all-round field of fire vital. Also, mounting the gun in a turret ensured that the tank could fire from behind some cover. Hull-mounted guns required that most of the vehicle be exposed to enemy fire. Multiple-turreted or multi-gun designs such as the British A9 Cruiser Mk I slowly became less common.