What is the relationship between the 6s in he number 6,647?

I'm not sure if you mean 6,647 as a decimal or as 6 thousand 647. I'll do it for both:

In a decimal of 6.647 the first six is for ones and the second six is for tenths, which means the first is 10 times as much as the second.

In a number 6647 the first six is for thousands and the second six is for hundreds. It also means the first is 10 times as much as the second.

The first digit is in the middle school 100s place