I have two Summary fields ("A" and "B") placed in a Part sub-summary when sorted by...." Each will give me the correct summary based on a specific category. So far so good but I can't develop a summary where I get one of the summaries subtracted from the other summary in the same Part (A - B). Can this be done?
You need to use the GetSummary ( summaryField ; breakField ) function. Its first parameter is another field of type Summary, and its second is the field that should be sorted on. You will need two those, with different first params, to do the subtraction.