Can you explain the Fitzpatrick skin scale?

For laser hair removal, I am confused with the Fitzpatrick skin scale because people of certain ethnicities could be VERY light or very dark. Additionally, someone could be mixed raced. They don’t all fit into a box!

What about a white person who is very brown? Such as a brown Italian. What skin type are they? III OR IV?

What about a mixed raced person? Black and white mix? Or even more complex, mixed with 3 races… such as Puerto Ricans (black, white, hispanic).

What about an Indian person with extremely fair skin that is as white as snow? Are they still skin type 5 or 4?