From NEC Section 505.9:
505.9 Suitability of Equipment.
(A) Suitability. Suitability of identified equipment shall be determined by one of the following:
Equipment listing or labeling
Evidence of equipment evaluation from a qualified testing laboratory or inspection agency concerned with product evaluation
Evidence acceptable to the authority having jurisdiction such as a manufacturer's self-evaluation or an owner's engineering judgment
FPN: Additional documentation for equipment may include certificates demonstrating compliance with applicable equipment standards, indicating special conditions of use, and other pertinent information.
Equipment that is listed for a Zone 0 location shall be permitted in a Zone 1 or Zone 2 location of the same gas or vapor, provided that it is installed in accordance with the requirements for the marked type of protection. Equipment that is listed for a Zone 1 location shall be permitted in a Zone 2 location of the same gas or vapor, provided that it is installed in accordance with the requirements for the marked type of protection.
Equipment shall be permitted to be listed for a specific gas or vapor, specific mixtures of gases or vapors, or any specific combination of gases or vapors.
FPN: One common example is equipment marked for “IIB. + H2.” (C) Marking. Equipment shall be marked in accordance with 505.9(C)(1) or (2).