How are pieces graded here in Piano Street?

[cnmrhyan]

I'm quite sure this has been answered before but after doing several searches I came up with no "concrete" answer. How is the music in here graded and what criteria is used?  I know there is a big difference between Level 1 and Level 8+  , but what about, let's say, Level 2 & 3? 

[ryanyee]

the technique required is a little harder and the fingerings required may also be a little more difficult.