Glossary
What is a rating on a pilot certificate?
An addition to a certificate naming what the holder may fly or the conditions they may fly in, such as airplane single engine land, or an instrument rating. A rating is earned through training and a practical test, and it is printed on the certificate.
That is the whole entry for now. The longer version, with the mechanics and worked examples, is still to be written. See autorotation for how those read once they exist.