An independent device is a heater that is not connected to a room in the Millheat app.
The independent devices will not follow a program as it is not connected to a room. In stead of following a program, it can be controlled through a timer function that makes it possible to turn the heater on and off.
All kinds of WiFi heaters can be used as independent devices, but the heater needs to be Generation 2.
Follow instructions in tutorial video below to assign an independent device back to a room: