Engine Shield Bonus. Higher tech engines boost the efficiency of shields.
Here's the entry from Donovan's.
The Engine Shield Bonus
When enabled in the host configuration, the type of engines a ship has can influence the strength of that ship in combat with other ships. When the ES Bonus is enabled, a configurable percentage of the cost of one engine is added to the ship's mass (note: only in battle, this does not influence the ship's normal mass and has no influence on fuelconsumption or damage taken from minehits and glory devices). When for example the ES Bonus is set to it's default 50%, a ship with transwarp engines (no matter how many) will have 50% of 300 (the cost of one engine) kilotons of mass added to it's battlemass. Such a ship effectively has 150 extra kilotons of battlemass, making it less vulnerable to impacting torpedoes, beams and fighter-hits. Keep in mind, the Engine Shield bonus only works in ship vs. ship combat.
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