Could you specify which textbook and which page? Thanks.
Boolean algebra deals solely in ones and zeroes. It has only three operators: addition, multiplication and negation. There can be no negative numbers. Namely, -1 is not an allowed number, so the concept of subtraction is meaningless is Boolean algebra.
Having said that, Y= ABCD – ABD has either a misprint – instead of +. In this case,
Y= ABCD + ABD = ABD(C+1)=ABD(1)=ABD