There are 4 different policy types to explore, each attributing to a different style of government. All types must be applied to an empty slot of the same type in the government, but a Wildcard slot can house any type of policy.
Military: These policies are best for players who want a strong military on land and sea.
Economic: These policies can be useful in increasing all sorts of yields, whether through increasing district adjacency bonuses or reducing costs.
Diplomatic: These policies are best for players with a strong interest in building relationships, and can help strengthen the bonds with city-states or improve espionage operations.
Wildcard: These policies accelerate the rate of Great Person production.