The DEKRA video on explosion protection clearly illustrates the topic.

The ATEX Directive 2014/34/EU, DEKRA and its conformity-assessment procedure are mandatory for any equipment and protective systems intended for use under the specified conditions in potentially explosive atmospheres operating at atmospheric pressure and temperatures between –20 °C and 80 °C, and at total pressures of 0.8 bar to 1.1 bar absolute. In this context, “equipment” covers all electrical and non-electrical apparatus that could act as an ignition source—such as lighting fixtures, control cabinets or electrical sensors that might ignite by sparks, arcs or hot surfaces. It also includes couplings, fans, compressors or rotary airlocks, where hot surfaces or sparks from rapidly moving metal parts may pose a risk.

Equipment requirements increase from Category 3 to Category 2/M2 up to Category 1/M1. For Category 3 devices, the manufacturer may declare conformity via internal production control. For Categories 2/M2 and 1/M1, a notified body must be involved, and its identification number must appear on the Ex-marking of the equipment. Conformity can be demonstrated either by individual testing or by an EU-type examination (Module B) combined with a certified quality-management system (e.g. ISO 9000) of the manufacturer. Chapter 2 of the ATEX Directive describes the possible module combinations, and its annexes give full details of each module.

The second group covered by the Directive is protective systems—examples include flame arrestors, which locally limit an explosion, or automatic suppression systems. Autonomous protective systems, like devices in Category 1/M1, must also be inspected by a notified body.

Any equipment or protective systems whose operating conditions (temperature/pressure) fall outside the ATEX scope must demonstrate explosion protection as part of the risk assessment under the German Industrial Safety Ordinance (BetrSichV).

Source: Wikipedia: Explosionsschutz