Temperature class (also known as ‘T-rating’) defines the explosion hazard type for the Ex product. The class, from 1 to 6, indicates the highest temperature allowed without igniting anything in the area. Class defines risks of a zone in a given area.
The temperature class schema was created to help simplify the discussion around safe temperatures in Ex areas. Think of it as a standardized communication protocol, like SMS for text messaging. If you have an area classified as T4, you know you can use T4 rated products.
The classes are defined separately for gas and dust hazards by IEC standards 600079-10-1 (gas) and 600079-2 (dust). The temperature restrictions for gas apply to the whole product, inside and out. The dust classes only concern the hottest outer surface temperature.
An important side note here: currently the standards do not cover areas where both gas and dust explosion risks are present.