What is ISR?
The ISR (Impuesto Sobre la Renta) taxes individuals' income. In employment, the employer withholds it each period by applying the monthly table published by the SAT (the tax authority), which is progressive.
Low incomes receive the subsidio para el empleo (employment subsidy), which reduces or eliminates the withholding. The calculation is adjusted every year.
In practice
With a salary of $20,000 a month, the income is placed in the matching bracket of the table and the fixed fee is added to the percentage on the excess over the lower limit.