Certain Dangerous Goods require mandatory transport in reefer containers due to safety reasons. These are some of the Self-reactive Substances under Class 4.1 and some of the Organic Peroxides, Class 5.2. This requirement is listed in column 16 of DG List in chapter 3.2 and respective sub sections in chapter 2.4 & 2.5 of IMDG Code (35-10).
So what goods which do not require a reefer is sometimes transported in reefer? Generally these are Perfumes ( UN 1266), Safety Matches ( UN 1944), Paint (UN 1263), Lithium Batteries (UN 3090,3091,3480,3481). Some shippers transport these in live reefers to maintain the product quality. Of course Calcium Hypochlorite require live reefers by most of the shipping lines due to its bad past.
IMDG Code has certain provisions for Dangerous Goods in reefers for commercial reasons, these are specifically related to Flammable Gases ( Class 2.1) and Flammable liquids ( Class 3). When Flammable liquids having flashpoint less than 23 °C c.c is transported in live reefer then
1/ Substance shall be precooled and
2/ Transported at a temperature at east 10 Deg C below its flashpoint
Or explosion proof reefer is required.
At any moment during transport In case the reefer fails then power supply shall be disconnected and must not be switched on again.
No line will accept flammable gas in live or non-live reefers. ( For full details refer 7.7 of IMDG Code)