Classification Abc - Margin model

Hi, I went I through the ABC classifications manual in AX 2012 and I would like to know how AX calculates the Margin ABC Model. I know that it differs on the type of item that we are going to take into consideration.

In particular, if it is a non stock item but it is category based, is it right to say that the margin is the difference between the Net invoice amount and the cost (that is 0) + tax amount in Brazil?