I found this chart to help us remember what setup fields are used in packing slips and invoices when posting purchase orders in AX 2012 under the new accounting framework. I believe some of the posting complexity is due to accommodating Standard Costing in addition to other costing methods. In addition download the whitepaper from Microsoft to understand the entries posted during PO, material receipt and Invoice for Stocked products.
The image is taken from this forum. DynamicsUser