The Missile Sub is the Soviets' long range naval weapon. It remains submerged unless shooting. When submerged, it cannot be seen unless it enters the detection radius of other nearby submarines, missile submarines, gunboats, or destroyers. It can shoot long-range missiles that reach both air and ground units. It is slow and weak in close combat, and needs to be protected by regular submarines.
Although the missile submarine can detect other submerged units, it cannot shoot them unless they surface.
The missile submarine looks very similar to the submarine, but it's not the same unit and visual differences exist.