ATEX derives its name from the French title of the 94/9/EC directive: Appareils destinés à être utilisés en ATmosphères EXplosibles... and the directive targets equipment and protective systems intended for use in potentially explosive atmosphere.
For a "Ex" coupling to be in certified... the coupling must under go a detailed process that includes a lengthy review of the design specification against a series of standards, and, in many cases, physical testing. ATEX couplings also have special labeling requirements.
As ATEX is a common directive across all EU countries, most major coupling companies (and those who do significant business in Europe) will be able to knowledgeably speak to your ATEX questions and/or provide ATEX certified product upon request.
In North America, requests and questions for ATEX couplings are primarily focused on products that will be exported to Europe... and/or replacing product in North America that were imported into North America with the designation labeling on it from somewhere else.
The Rubber Manufacturers Association and Mechanical Power Transmission Association have released a technical bulletin with a bit more detail and explanation of the directive should you be interested in learning more.
Recommended Follow-On Reading: To better understand coupling interchanges and risks of switching between different coupling manufacturers, check out: Coupling Interchanges - Should You Mix And Match Different Manufacturers?