Power allows you to control appliances that are plugged into Bye Bye Standby sockets. Please note that in addition to the sockets themselves, an Online Energy Controller is required.
Your iPhone or iPad must be in the same IP broadcast domain as the Online Energy Controller, since the app works by broadcasting UDP packets to the controller. Typically this will automatically be the case if your online controller is plugged into a wired port of your home router, and the iPhone/iPad is on a WiFi network provided by the router. It is not necessary for the controller to be registered with a web-based service to work with this app.
The Bye Bye Standby sockets and Online Energy Controller all operate on the 433 MHz band. As such it is possible that many other brands of socket (Home Easy, Domia, Brennenstuhl et al.) may be controllable via the app and controller. It is also possible, although perhaps less likely, that other controllers may work but please note that neither of these scenarios has been tested.