The changes in NH3’s flammability ratings in NFPA’s 704 ratings comes down to the difference between ammonia’s inherent chemical properties and specific regulatory overrides designed for enclosed refrigeration systems.
Strictly speaking, the base chemical rating for pure anhydrous ammonia is a “1”. However, the International Mechanical Code (IMC) and the International Institute of Ammonia Refrigeration (IIAR) mandate a rating of “3” for indoor machinery rooms to account for the elevated risks of confined spaces and entrained oils.
Here is how the codes map out the hazard: