Heavy Assault Missiles are a type of self-propelled, self-guided weapon. They operate similarly to Torpedos but are limited to sub-luminar speed.
The difference between Missiles and Torpedoes is subtle and, depending on the manifacturer, paper-thin. In general anything that doesn't have a FTL drive is called a Missile, and anything that does is called a Torpedo. For example 1000 years ago, in the Alfa Quadrant of the Milky Way, the Leyron Republic used Missiles that could be attached to a warp unit transforming them into Torpedos; depending on the situation they would decide what they wanted to fire and thus not waste precious warp drives when shooting at sub-luminar speed. The Leyron term they used to call them should be translated "hard-shoot-travel".
For no reason a Missile should be in general more powerful, or slower, than a Torpedo. Subluminar reach, explosive power, targeting ability and esthetic qualities can be equal. Torpedos tend however to be more expensive as they contain a FTL drive which consumes fuel.
Heavy Assault Missiles are a kind of Missile that happens to be slower and more powerful than a Torpedo and are sometimes used in warfare in the Triangulum Galaxy. "Heavy" signifies that they are bigger and contain more explosive material, and "Assault" probably is a way to say that they are not used to help agricolture nor to write poetry (the ode Oh Missile my Beautiful Missile by Slugrian general Stoh-Kahz being the only recorded ecception).