Final answer:
A proportional tax rate system is a flat tax rate structure where everyone pays the same percentage of their income as tax, unlike the progressive tax system used in the U.S. where higher earners pay a higher percentage.
Step-by-step explanation:
A proportional tax rate system is best described as a flat tax rate structure. This means that everyone pays the same percentage of their income, regardless of how much they earn. For instance, if the proportional tax rate is 10%, a person earning $100,000 would pay $10,000 in taxes, and a person earning $10,000 would pay $1,000. This is different from a progressive tax system where higher earners pay a greater percentage of their income in tax, and a regressive tax system where higher earners pay a smaller percentage of their income in tax.
The U.S. federal income tax system is actually a progressive tax system, not a proportional one, because the rate of taxation increases as an individual's income increases.