How to wire AC power to an integrated DPST switch?
Both contacts have same rating so unless there is a designated marking or colored screws for convenience,  they can be interchanged. 

In some countries this universal PSU could be coming off a Y or delta 3 phase output rather than single phase to neutral. So DPST is universal but not needed for single phase.

Also minor current comes off unswitched line due to Common mode filter Cap  to ground. (0.5mA rms max)