The paper industry uses two factors to determine the weight of a quantity of sheet stock: Basis Size and Basis Weight.
Each paper type has a Basis Size, as follows:
Offset and book papers: 25"x38"
Cover: 20"x26"
Index: 25.5"x30.5"
Bond: 17"x22"
Bristol: 22"x28"
Tag: 24"x36"
By definition, Basis Weight (BWT) is the weight of 500 sheets (one ream) of Basis Size paper. Therefore, 500 sheets of 25x38-60 BWT offset paper would weigh 60 lbs.
Within the context of the Enterprise program, Basic Area is just the area corresponding to the Basis Size.