Image Subtraction question

[Budric B]

Hi, I have a simple question for those with experience. A part of an algorithm describes to do a difference between image, A-B. My question is what if A[i, j] < B [i, j] for one or more pixel i, j? Negative intensity values are meaningless. Is there a standard definition for image subtraction? I can't seem to find an answer to this simple question. Thanks.