Ao pesquisar entendi que essa é uma forma de pagamento de impostos que leva em consideração a receita anual do contribuinte.

Salaried employees normally have part of their paychecks withheld for taxes. However, many U.S. taxpayers, such as the self-employed, must make lump sum installment payments of some or all of their estimated taxes. The Internal Revenue Service offers three methods to figure installment payments. The annualized income installment method, or Schedule AI, is one such procedure that is useful to taxpayers who receive varying amounts of income from period to period. The installment payment periods end on the 15th of April, June, September and January.