The mark has three parts: explosion-proof form (Ex d flameproof, Ex e increased safety), gas group (IIA/IIB/IIC, IIC strongest) and temperature class (T1-T6, higher number = lower allowed surface temperature). For example Ex d IIB T4 means flameproof structure, IIB gas, surface temperature not above 135°C. Choose a rating no lower than the actual site hazard and check the certificate validity.