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The ModifierType

The Subject, as well as what exactly a modifier does, are determined by the ModifierType. A ModifierType is a combination of an EffectType and a CollectionType. The Effect type determines what it does, and the collection determines who it affects. We know enough at this point to start making a new modifier in earnest, so let’s create a modifier called “Irrigation Ditches” as an example, to use with policy card of the same name. The code below creates a new ModifierType that combines the EFFECT_CITY_ADJUST_YIELD effect and COLLECTION_PLAYER_CITIES. So when applied, it will try to adjust City yields in all of a specific Player’s Cities. 
INSERT INTO Types
        (Type,                                      Kind)
VALUES  ('FXS_IRRIGATION_DITCHES_MOD_TYPE',         'KIND_MODIFIER');

INSERT INTO DynamicModifiers
    (
        ModifierType,
        CollectionType,
        EffectType
    )
VALUES
    (
        'FXS_IRRIGATION_DITCHES_MOD_TYPE',
        'COLLECTION_PLAYER_CITIES',
        'EFFECT_CITY_ADJUST_YIELD'
    );
We create a new modifier with the FXS_IRRIGATION_DITCHES_MOD_TYPE ModifierType. We can then assign it to the TraditionModifiers (which are the modifiers for Traditions and Policies), so when a player slots the Tradition into their government, the yields of all their cities are modified. 
INSERT INTO Modifiers
    (
        ModifierId,
        ModifierType
    )
VALUES
    (
        'FXS_IRRIGATION_DITCHES_MODIFIER',
        'FXS_IRRIGATION_DITCHES_MOD_TYPE'
    );

INSERT INTO TraditionModifiers
        (
            TraditionType,
            ModifierId
        )
VALUES    (
            'TRADITION_FXS_IRRIGATION_DITCHES',
            'FXS_IRRIGATION_DITCHES_MODIFIER'
        );

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ModifierArguments

Neat, but what type of yields are we adjusting here? While a ModifierType determines what sort of effect a modifier has, entries in the ModifierArguments table control the specifics of the effect. So for example, let’s tell the modifier we just made that we want to adjust the yields specifically by 5 Food! 
INSERT INTO ModifierArguments
    (
        ModifierId,
        Name,
        Value
    )
VALUES
    (
        'FXS_IRRIGATION_DITCHES_MODIFIER',
        'YieldType',
        'YIELD_FOOD'
    ),
    (
        'FXS_IRRIGATION_DITCHES_MODIFIER',
        'Amount',
        5
    );
ModifierArguments consist of a Name, a Value, and the ModifierId of the modifier it controls. Which argument Names are available will vary based on the EffectType, and the easiest way to figure that out is to look for other modifiers that share the same EffectType in the game files or through gameplay-copy.sqlite in the Debug folder. Putting this all together we have a modifier attached to a TRADITION_FXS_IRRIGATION_DITCHES tradition—which technically still needs to be made—that grants +5 Food per turn to all of a specific player’s cities. Continue to: Modifier System — Requirements and GameEffects XML