I think it must be setting them up for addition and subtraction when they carry/borrow, so for example you can borrow a box and it turns into loose items. That is what I remember DS doing (sorry to tell you that I think you'll have to endure the "packing boxes" for a while longer, just in different formats).
It gets worse because they have to fill in values for carrying/borrowing in a bizarre sort of fashion and it actually makes more sense just doing it on paper. I could never figure out what they were trying to get him to do and left him to his own devices. I'm glad he seems to be done with it.
