If you perform energy models, you will almost certainly hear a reference to ASHRAE 62.1 at some point.  For energy models, it is primarily used to determine the ventilation provided to spaces based on the space type (often the square footage and the peak occupancy of the space). However, the standard covers much more than that.

It is typically required in ASHRAE 90.1 or LEED certification, UNLESS a local code is more strict, ie. the local code requires MORE outside air than that of Standard 62.1

ASHRAE Standard 62.1, also known as Standard 62.1 or just 62.1, is a standard for ventilation and acceptable indoor air quality in buildings. It is published by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) and provides guidelines for designing and operating ventilation systems that deliver clean, fresh air to building occupants while minimizing energy use.

Standard 62.1 covers a wide range of topics related to indoor air quality, including ventilation rates, air cleaning and filtration, and the use of outdoor air for ventilation. It also includes guidelines for controlling pollutants such as tobacco smoke, radon, and volatile organic compounds (VOCs) that can be harmful to human health.

The standard is updated every three years to incorporate new research on indoor air quality and ventilation. It is widely adopted by building codes and is used as a reference in the design and operation of ventilation systems in commercial, industrial, and residential buildings.