Hi Andy,
It is used as a variable string label in the Custom buttons of the controller bar and the show notes screen. It allows you to see the value of a controller you are adjusting in real time, like a slider or expression pedal or in this case a pitch wheel value.
To use the variables you route the desired pedal, wheel or slider CC values to the Onscreen Keyboard output port so it can be read and displayed by the string variables you placed in the appropriate text boxes of the Show Notes or the Controller Bar buttons labels.
The basics of the variables and syntax are pointed to at this link.
Cheers,
Dave