What do AU, LO, MD, and HI stand for? June 30, 2023 01:58 Updated These are the letter symbols for the Gradient unit’s fan speed. AU = Auto LO = Low MD = Medium HI = High Related articles Why does the indicator ring change in size? What is the indicator ring on the top of the dial? What are modes? How do I use the Comfort Control Dial? Just how easy is it to install the Gradient?