7 ply string?

I’m still a bit confused with string terminology but has anyone ever used, seen or even heard of 7 ply yoyo string? I’ve seen 4, 5, 6, 8, 9 and 10 ply, but where’s the 7 at?! I ended up making some myself and have started using them all the time, I don’t understand why I can’t anything on the internet even mentioning them.