Certain ship types and PDBs have been placed into categories that gives them bonuses against another category. This was added to the game system to promote the development of mixed fleets and defenses.
Ships of the Line (SoL) – Dreadnought, Battleship
Cruisers – Heavy Cruiser, Light Cruiser
Escorts – Destroyer, Frigate, Corvette
Civilian – Colony Ship, Small Cargo, Large Cargo, Salvage Ship
Civilian ships do not get any combat bonuses and are easily detected and destroyed. All other ship types have their own special missions and do not get combat bonuses. During combat, there is a chance that an escort class ship will intercede when a civilian ship is being attacked.
SoL get a to hit bonus against the Cruisers category.
Cruisers get a to hit bonus against the Escorts category.
Escorts get a to hit bonus against the SoL category.
As for PDB versus ships, while we don't have historical categories to draw upon, we have created categories similar to those for ships.
Heavy PDB – Orbital Defense Base, Missile Platform, Quark Beam Artillery
Medium PDB – Dark Matter Battery, Plasma Cannon, Ion Cannon
Light PDB – Microwave Lance, Laser Cannon, Rail Gun, Defensive Satellite
SoL get a to hit bonus against the Medium PDB category.
Cruisers get a to hit bonus against the Light PDB category.
Escorts get a to hit bonus against the Heavy PDB category.
Heavy PDB get a to hit bonus against the SoL category.
Medium PDB get a to hit bonus against the Cruisers category.
Light PDB get a to hit bonus against the Escorts category.
The combat class of ships also add bonuses when performing mission types.
Recon
ships get a fleet detection or avoidance bonus depending on their role.
Corvettes
get a to hit bonus against all ships and PDBs while in Planetary Defense position. As an escort class ship,
there is a chance this ship will intercede when a civilian ship is being attacked.
Frigates
improve the rewards when on Raid operations. As an escort class ship,
there is a chance this ship will intercede when a civilian ship is being attacked.
Destroyers
will get a bonus by being the fastest combat ship. As an escort class ship, there is a chance this ship will intercede when a civilian ship is being attacked.
Light Cruisers
improve the chances of ground combat during Genocide operations.
Heavy Cruisers
improve the chances of ground combat during Invasion operations.
Battleships
increase the chances for a fleet to hit a target during Bombardment operations.