A unit reaches Heroic status it has reached the maximum unit rank after veteran status. Once a unit becomes heroic it gain access to better armament and support tools (such as repairing kit or medikit) to enable them survive longer in the battlefield. Units experience is expanded as they have master their weaponry with better result.
Whilst fighting for your comrades, your freedom or for the Emperor. You would generally make a good impression of slaying enemies continously but in the end, a superior foe would mean the buzz-kill.
However, some pilots and soldiers would have sheer luck to escape their perils most of the time and all three factions have devised a rank system for those who would push their luck on what would-be suicidal missions.
The highest of course is Heroic. When a battle-hardened soldier, or vehicle pilot, does what he is meant to do more properly, he'll find himself in the ranks of Heroic. Now, these heros of war are more brutal than the average grunt that walks out, fresh from the Barracks. Combined with their inital training and their battle-hardened experience, Heroic units are 3x tougher to kill, as they know true pain. As a result of this, the Heroic unit can dish out triple the damage as well, meaning a normal unit will stand no chance against another with such augmentations.
As well as enduring and dishing out the same amount of damage, Heroic units heal themselves in combat and also are capable of firing much quicker, making it harder for units to escape their approaching fate. With this in mind, Heroic units like to show off by adding a little bit of red to their ammo. Indicating the potential threat, these heroes possess.
This means that a Tank would generally best 2-3 unranked counterparts or challenging the tougher bruisers up the food chain or a 1-2 Heroic Bullfrogs downing even the resilient Century Bomber to solid ground.
Heroic Units can still be overwhelmed with superior numbers or, more efficently, use their weaknesses to great effect. Luckily though, units that destroy a heroic enemy gain a boost in rank, possibly to keep up its morale and keep it confident.
Heroic Units can be plainly seen with gold stars next to their health bar as well as their weapons projecting red highlights to their ammo.