[UserNameGoesHere]

Single. Digit. Good.

Why are so many people voting for that third option?

It's pretty clear it is exactly as it says.

I will enumerate. SDG is inclusive of the following and exclusive of everything else
1-0-0-0
2-0-0-0
3-0-0-0
4-0-0-0
5-0-0-0
6-0-0-0
7-0-0-0
8-0-0-0
9-0-0-0

There.
10-0-0-0 is not an SDG. Nor is 1-0-0-1 or 3-1-0-0 or anything else.