EMP is a type of damage dealt by some weapons, most notably ion weapons. It deals zero damage to shields, armor & hull but instead directly builds up damage again weapons and engines if it is not blocked by a shield. This can be extremely effective at disabling weapons & engines. Certain weapons additionally have the ability to sent ion arcs across ships they hit, disabling weapons and engines that were not directly hit. The Ion Cannon & Ion Pulser can only create arcs when hitting hull, but the Ion Beam & Tachyon Lance can create arcs when hitting shields, with a greater chance of piercing if the target has high hard flux. The Mjolnir Cannon & Hypervelocity Driver do not create EMP arcs.
Actual effect is somewhat inexact as the EMP damage is proportional to the distance between point of impact and the weapon or engine, believed to be similar to the damage pattern for armor cells, except it does half-damage to the primary/secondary cells and quarter damage to tertiary cells.
This means that broadly speaking weapons set further into the hull will take less EMP damage as they will likely be further from the point of impact. Some niche cases exist where the point of impact may be in the middle of the ship, such as Ion Cannon armed Claw or Thunder Multirole Fighter Wing firing during overflight.
Weapon durability is 250/500/800 for small/medium/large turrets. Doubled if a hard point instead of a turret. Potentially doubled by Armored Weapon Mounts. Mounts defined as Hidden cannot be disabled. Engine durability is based on hull size and size of the engine plume, with bigger engines on bigger ships being more durable.
Exact engine durability is a baseHP of 100/200/400/600/800 for fighter/frigate/destroyer/cruiser/capital and the following formula, where "size" is engine glow width + height, clamped to [0, 100]
maxHp = baseHP * (0.75 + (size/100 * 0.5))