The mouse reference is a pointer. If it's NULL, the driver uses a default. If it's non-NULL, then it points to a function that the driver can call. That function will adjust the driver's calibration value. It could ask the user to adjust the pen; or, it could read a file that holds a value from a previous calibration.
Application writers can choose how it's done: a function that's provided by the library, a custom function, or nothing.
