Just a quick question regarding grain usage in home-brewing:
Used at the rate of 3 to 15 %
What does this refer to?
That means you would use it at 3-15% of your total grain bill.
Let's say you had a recipe that used used 10lbs of grain in total. They are recommending that you wouldn't want more than 4.8oz-1.5lb of this amount to be Biscuit Malt.
They suggest these limits, because the flavors imparted by these grains can possibly overwhelm other components of the beer. That said, if you find a recipe that sounds good that uses a malt like this in higher proportion than suggested - brew on!