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I think Nut covered it. As far as I know, they both use the same scoring system. However, P&Y is strictly archery whereas I think any legal weapon can be used in entering an animal in B&C. I also know B&C lists animals that are found dead.
The let-off is the percentage the poundage of the bow lessens after you come to full draw. When you draw a compound notice how it gets harder and then as you get to full-draw it get easier. That is let-off.