How does ground combat work, exactly?

[Zereth]

What does the Ground Combat racial trait do, exaclty? Is the Ground Combat Damage Modifier Percent added to damage from ground combat weaposn, or subtracted, or is that the percent of the weapon's listed damage they acutally do? Does this apply to the damage reduction from Small Emissive Armor as well? Is there any point in adding Combat Sensors or ECM to your troops, or should you jsut load them up with weapons and armor/shields?