HEAT in the game is for
High Explosive Antitank, as in shaped charge.
This kind of ammo is used against armored targets, can also be used against soft vehicles and infantry when no HE is available, but only for unguided weapons as far as I know.
There is no difference made from the type of guidance in missiles in any case. And anyhow, of Fagot and Strela, only the Strela is heat-seeking, and you'll see that it comes only with HE ammo.
Just nitpicking
