I don't understand what the difference in modeling between: Entry(account, direction, non-negative amount), direction is debit or credit. vs Entry(account, s

I don't understand what the difference in modeling between: Entry(account, direction, non-negative amount), direction is debit or credit. vs Entry(account, signed amount), + is debit, - is credit (for example). It's a two way mapping and should be equivalent. Unless debit or credit amount could be negative. But as I understand it's a big NO-NO in accounting.