Library and Loops

I would like to understand the organization on garageband library and its loops (and the same in MainStage).

To give a little bit background I have garageband and I reinstalled one jam pack that I had onDVD. I wanted another jam pack and I found that the cheapest way to get it was to buy MainStage. I bought it, installed it, got all additional ant I think all jam pack seem OK both in MainStage and in garageband. Now I want to explore it and find it is difficult without understanding the logic of it.