Fitzpatrick is a numerical scale that measures the amount of melanin in the skin after exposure to the sun. The scale was developed to measure how skin reacts to ultraviolet light, or sun exposure.
https://www.medicalnewstoday.com/articles/320639#how-to-protect-each-type
The link above is highly educational, and it is recommended to read through in detail to gain better understanding.
There are several websites which assist in providing guidelines as to which type you or your clients is classified. Below are a few reliable links;
https://www.westdermatology.com/news/find-your-fitzpatrick-skin-type
https://lazaderm.com/fitzpatrick-test-identifying-your-skin-type/
