Ask HN: What are the differences between co founder shares and class A?

I'm in a situation where I have to decide what's best for me. I'm the third member of a co founding team, or so I thought. I was offered class A shares, and am not sure what's the difference. They inform me that I have equal voting rights, but I don't understand why I can't get co founder status as well.

This might be a dup, couldn't find my original thread