The document discusses different models for measuring gross domestic product (GDP):
1) Two sector models include households and farms, with households receiving income from farms in the form of wages, interest, rent, and profit and spending that income on farm goods and services.
2) Three sector models add the government sector, which collects taxes from households and businesses and spends on goods and services.
3) Four sector models account for international trade by including exports and imports in GDP calculations. Exports are spending on domestic goods and services by foreign entities, while imports subtract foreign goods and services purchased by domestic entities.