This is a separate unit that can be removed from the main gearbox and is often the root cause of gearbox and drive issues.
The mechatronics unit deals with gear selection and is a complex unit with its own oil and its own Transmission Control Module (TCM). The unit contains its own oil supply (separate from the main gearbox) and includes slave cylinders, pump and high-pressure accumulators, maintaining an oil pressure in excess of 40 bar (your car tyres might be a typical 2.2 bar).
A common fault is an ingress of Sulphur from the main gearbox causing degradation of the mechatronics unit and consequently penetrating the TCM, causing electrical faults which confuse the gear selection process. In this case, you need a replacement mechatronics unit and not a whole gearbox and mechatronics unit combined.