Like ASHRAE standard 34, standard ISO 817 “Refrigerants —Designation and safety classification” provides an unambiguous system for assigning designations to refrigerants’ toxicity and flammability.
In this case, A2L has lower flammability and toxicity compared to other classifications—making it the second safest refrigerant category.
A = Non-toxic
2 = Flammable
L = Low burning velocity