A simple apparatus is a term applied to equipment being used in a hazardous environment. A simple apparatus by its very nature is incapable of storing electrical energy (as opposed to a battery, capacitor, or inductor). A limit switch with make or break contacts for example is a simple apparatus. Any electric potential is entirely defined by the equipment on the other end, such as an intrinsically safe barrier.
The US NEC code has no specific requirements for use of a simple apparatus in a hazardous environment, as its electrical potential is entirely defined by the equipment on the other end. For all practical purposes you could use a wire nut to connect two wires provided a proper intrinsically safe barrier is what powers it. Of course we do better than that. More common simple apparatus are devices such as limit switches, pressure switches, temperature switches, etc.
ATEX used to view simple apparatus just like the US code. However that has changed recently. Now even a simple apparatus requires ATEX certification. This basically means anything electrical being used in a hazardous area requires an ATEX certification.