It's overriding an unwanted reflection. To know which reflection shades nuance each unwanted reflection, we must base ourselves on the laws of colorimetry and Oswald's star.
For example:
The golden reflection (3) is nuanced with an iridescent reflection (2).
The copper reflection (4) is nuanced with an ashy reflection (1).
The reddish reflection (6) is nuanced with a matte reflection (7).
It should be nuanced when the person is not satisfied with the reflection of their hair.
In order to recommend the right tint, it is very important to know and diagnose the hair and the color of the reflection that you want to tint very well, this can only be done properly by an experienced beauty professional.