Well, the simple answer is that it stands for both and whilst there are minor differences they are used interchangeably in normal industry conversation.
Other meanings for RTU:
Returned to Unit - military: a member of the military being returned to their home base/unit, either due to being medically unfit, unable to complete training, or disorderly conduct.
Rooftop Unit - HVAC: an air handler usually located on the top of a building.
Real Time Ultrasound - medical: an imaging technique allowing visualisation of muscle contractions.
Remote Titration Unit - chemical: one for all you chemists and water analysis people
Radio Telemetry Unit - IoT: as above
Real Time UNIX - IT: couldn't resist including this one!
Feel free to add your favourites!