Is the difference between A and B A minus B or B minus A?

Answer: Most likely there is always difference. Often A and B are describing unknown numbers.

For example if A=10 and B=5.

A-B = 10-5 = 5

B-A = 5-10 = -5

Answer: It can be any of these (most likely A - B), or it can be the absolute value of A - B (which is the same as the absolute value of B - A).