The criteria are specified for a variety of reasons including for the protection of human health, buildings, crops, vegetation, ecosystems, as well as for planning and other purposes.
There is no internationally accepted definition but usually "standards" have some legal or enforcement aspect, whereas "guidelines" may not be backed by laws.
It is important when complying with ambient air quality criteria to check the direct reference as well.
These agencies also list whether criteria must be monitored in a specific way to fit with compliance measures.
[1] Annual arithmetic mean of a minimum of 104 measurements in a year at a particular site, such as taken twice weekly as uniform 24-hour samples.