The 'Mechatronics Uint' is basically the tranny's brain. It holds details of the type of vehicle and the gearbox shift pattern 'map' in the same way that the ECU holds the engine's performance map.

It also has various hydraulic control valves and solenoids that provide the feeds to the clutches etc inside the tranny that are used to actually command each shift change.

It also communicates with the engine's control systems to try to ensure smooth shifts during warm-up, normal driving and 'Sport' mode driving. I think it also holds the 'adaption' data for the drivers own style to improve/refine the shift patterns.

It sits in the bottom of the tranny just inside the oil pan. It's also expensive if it fails. Mine was £1200 plus fitting and was replaced following a catastrophic failure in the torque comverter which wiped the tranny and MU out - the MU due to metal fragments in the oil damaging the solenoids and hydraulics.

__________________
Me:
Current: 2011 E71 40D Vermilion Red/Beige Nappa

Wifey:
Current: 2012 R80 Countryman SDX Green/Cream