The Kei/Yuri factor had relevance for planetary scans.
The Kei/Yuri factor of a plane of frozen methane on the surface of Theta 116 VIII was 12×10-3. (TNG: "The Royale")
The Kei/Yuri factor was not mentioned in dialog or the script, but shown on an okudagram on a bridge viewscreen. It is an in-joke reference to the Japanese series Dirty Pair, whose main protagonists were Kei and Yuri.