That seems a bit more like a Forest Pro Pack forum question, but I'll do my best:
From my understanding, it's pretty straight forward... It works only when you have 2 or more items to be scattered (perhaps you played with it with only 1 scatter object?).
It groups them in 'clusters' - a size of your choosing (x). If you have grass types 1, 2, 3 it will 'group' them in bunches by type/item. So a group of grass type 1 in x radius etc etc.
Does this help?