The Z Key on FLX and FLX S consoles places a range of quick access settings & functions on the internal touchscreen and encoder wheels. It can be pressed at any time unless you are in Setup, and the LED in the button will light when you are in the Z window.
On FLX the Z window contains Quick Access Functions, Quick Access Settings, Other Windows, and also a handy clock top right of the window. There is also an "Error Tone On/Off" button, which can be used to turn on/off the built in error tone which can sound every time there is a popup, warning or error.
On FLX S consoles the Z window contains Quick Access Functions, Other Windows, and also an onscreen syntax number pad. FLX S48 consoles also have a button to allow you to calibrate external touchscreen monitors.