Understanding your own organizational data and they applying that to an ERP package can be something your spend a lot of time iterating on. I’d recommend understanding the data structures in the product at the same time you start your analysis of your own business processes. Both our understand and how you want to use the product and your understanding of your business processes will grow along the way. This video gives a quick run through of some of the legal entity concepts in Dynamics AX.
You might also be interested in some of the other supply chain organization concept I posted in a earlier post.