Radar Number notes....

I believe that there are many numist fans out there that collect some radar number notes for fun, or even as their main theme of collection.

I have yet to really dig myself into this particular theme but recent acquisition of few Radar no notes at face values prompted me to look for method differentiate them accordingly. Has anyone ever categorises them accordingly? Is there any special guide to follow?

The number of Radar no notes per run of 10 million notes gives the following results:
4-digit radars: 10x9x8x7 = 5040
3-digit radars: 10x9x8x6 = 4320
2-digit radars: 10x9x7   =  630
Solid radars:   10-1*    =   9
Total radars and solids:   9999

The total number of radar and solids in 10 million per run is astonishingly a lot.

Can someone out there share some light in this matter?