Long story short, it doubles the effects of all active corporations in the city.
A mining corporation that adds 8 production will now give 16.
Most complicated part is formulating the formulae in computing so if any of these is wrong, please tell me.
Gold:
A = Commerce from the Corp itself
B = Commerce Modifier due to buildings. Eg Bank
Amount Added to Player's Gold = A x (100 +B)/100
Research:
A = Commerce from the Corp itself
B = Commerce Modifier due to buildings. Eg Library
Amount Added to Current Research = A x (100 +B)/100
Culture:
A = Commerce from the Corp itself
B = Commerce Modifier due to buildings. Eg Cathedral
Amount Added to City's Culture = A x (100 +B)/100
Food:
A = Yield from the Corp itself
B = Yield Modifier, However, there isn't any building that increases food by % in BTS so not used.
Amount Added to City's Food = A
Production:
A = Yield from the Corp itself
B = Yield Modifier. Eg Forge
C = Iron Modifier
D = Coal Modifier. Thanks to that damn Ironforge. For mods where there are other resources giving production boost %, I provided a loop commented away
E = Other Modifier. Industrious Trait, Theocracy, Resource Bonus when building wonders, Unit Bonus with Drydocks, Military Academy Etc.
Amount Added to City's Production = A + (100 + B + C + D + E )/100
Production is the big headache.