fx12 is a free Android application developed by Favionics. It serves as the companion app for fx12, fxRDT, and fxWS devices, providing an interface to view and change parameters of fx12 and fxRDT, including altimeter and GPS data in real-time.

One important thing to note is that this app may use the user's location even when it is not open, which can potentially decrease device battery life. Continued use of GPS running in the background can significantly affect the battery life of the device. Overall, fx12 is a useful utility and tool for those who own fxRDT and fxWS devices and need to monitor and adjust their parameters on the go.