How to understand 17 gray scale in photography?

You are talking about the standard photometric version, which should be 18% gray scale, that is, "standard gray board". The surface reflection coefficient reaches 18%, which refers to the dark gray card board. Most subjects have various reflection coefficients and colors, and their average value is considered as 18% of the reflection coefficient, which is used as the benchmark for all tools used in conjunction with exposure. In the case of using reflective exposure meter and TTL photometry, if the standard reflector is used for photometry, even if there is a big difference between the brightness of the subject and the background, the photometry value will not be greatly deviated, which is more convenient.