to add to cracked butt: bdl's feature either a detachable mag or a hinged floorplate.
i don't think there is a person on this earth that could tell you exactly what adl and bdl stand for, and why that was the derivative. just know that bdl's are more expensive than adl's, and most of the difference is cosmetic (assuming identical calibers, and the adl's caliber line-up is awful thin these days).