ECU Tuning
The ECU controls engine behaviour: boost, torque request, fuelling, ignition timing and many safety strategies. ECU remapping is what usually creates the power and torque gain.
TCU Tuning
The TCU controls automatic gearbox behaviour. It can affect shift timing, torque limits, clutch pressure and how quickly the gearbox responds.
When You Need Both
If an automatic gearbox limits torque or shifts too early after an ECU remap, TCU tuning may let the drivetrain use the engine calibration properly.
What To Ask For
Ask whether your specific ECU and TCU are supported. Support depends on the vehicle, gearbox type and software version.