I tried modding the ship roles yesterday and realized, that even though I modified <Role> in the ShipBlueprintDefs and <AIShipClass> in the ShipClassDefs, the roles of ships were assigned consistently with:

http://galciv3.gamepedia.com/Ship_roles

"A ship's default role is determined by the attributes "THREAT", "FORTITUDE" and "VALUE". All ship equipment has at least one of these attributes assigned to them. THREAT is assigned to weapons (beams, kinetics, missiles), FORTITUDE is assigned to defenses (hull plating, shields, point defense), and VALUE is assigned to other equipment (drives, life support, modules).

As equipment is added to a ship the sum of these attributes are added together to decide what role the ship will have. If THREAT is the largest it will be an attacker, if FORTITUDE is largest it will be a defender, and if VALUE is larger than the sum of THREAT and FORTITUDE the ship is labelled as support.

The hull size also influences what role a ship will have, though all hull types with VALUE as the highest will be Support. Tiny ships can be Interceptors (THREAT) or Guardians (FORTITUDE), small ships can be Assaults (THREAT) or Escorts (FORTITUDE), medium and large ships can be Capitals (THREAT) or Escorts (FORTITUDE) and huge ships will be Capitals regardless of whether THREAT is larger than FORTITUDE."

Meaning I got support carriers and no escort huge hulls for example.

I was trying to make the AI use capships as damage dealers, escorts as tanks and guardians for support modules and carriers. Assaults and interceptors would only be assigned to smalls and tinies.

I checked for the same on the AI Tweaks mod (maybe a little old version) and found the same result: The settings above do not influence the blueprints you can see the AI or the player uses.

Is this a bug, is there a way to overwrite the roles ships are assigned? Are the assignment rules (influence of threat, fortitude and value) in the moddable xml files?