What is the differences between the account assignment category of E and M?

Question

Hi, I have a question about account assignment category.I hope someone give me the answer , and I would appreciate it.
    as you know, there are so many account assignment categories (e.g. A:asset,K:cost center)in sap customizing,when we want to purchase asset, we will input A to the field of account assignment category in purchase order.
     When will we input E or M in purchase order? What will it imply when we input E or M in purchase order? What is the differences between the account assignment category of E and M?
Best Regards
Bob Wang

Answer 1

Vijay,Thanks for your answer,but I am still not clear about it,I have a instance to describe my question at detail.
    If there is a final product A, and the planning strategy for A is 20(MTO), the procurement type of A is F(external procurement) at the view mrp2 in the material master datas.
step1: I creat a sales order.
step2: run MRP for A.
step3: transfer the purchase requistion into a purchase order,and the field of acc.***.cat. in the purchase order will be filled out M automatically, because the acc.***.cat. in the planning strategy 20(MTO) is set with E.
    Well, the purchase order is created, what is the relationship between the sales order and the purchase order? What will be happened about costing between the SO and the PO?
    If I delete the E, I make the PO become a standard PO, what is difference between standard PO and the PO including E?
Best Regards
Bob