The number represents the fat content in it, so the higher the content the creamier it is.
The standard used to be 3.6, but now there is 3.7 which is a 0.1 increase in fat content.
At Yumemiya they now serve 3.7 Hokkaido milk softserve and there are lots of choices as well as original and green tea.
Hokkaido 3.7 milk green tea and vanilla icecream:
Compared to other softserves in Hong Kong, this one is definately the creamiest if you have not tried softserves in other countries.
For the green tea part, it tasted too grassy.