Who is subject to the ISO and can it be credited against income tax?

The Solidarity Tax (ISO) is owed by those carrying out commercial or agricultural activities with a gross margin above 4% of gross income. The ISO is creditable against income tax; any balance not credited within three years becomes a deductible expense, and there is no refund of excess amounts. These rules come from PwC and from summaries of the law (leyestributariasguatemala.com, leydeguatemala.com); they were not checked article by article against the official text.