What is it used for Default Supply area in transaction MM01-MM03; MRP 2 view?

Assuming your customer wants to see the issued materials for production with movement type 261.

Production time is long and production planning is changing frequently. So they want to see the materials not used and wait for production although they are issued. Is it possible to see these materials elsewhere? (no Kanban and JIT is used)

Well, the supply area is the logical for staging the material to be used in production.  Normally, supply area is linked to the work center used in the operation of the order. All componentes assigned to this operation will be using the supply area related to the WC. In fact, all WM staging is made in connection to this area and stock balance is made to define if new issues from warehouse have to be carried out .

You must check if this default supply area overrides somehow the supply area assigned to the material thru the WC.

Of course, all these funtionalities are valid under the WM system, so if you are using only Storage locations, only reservation can be seen (unless you make an additional movement) 

If you use WM, you can trigger TR created via Production order, then when you delivery material from warehouse to supply area, you convert the TR to TO, and you can know the open quantity from TR item details, therefore you know how many material you have transfer to the supply area against the production order; on the other way, you can use MB51 to list display all the goods issues quantity against the production order, you can compare the goods transfer qty and goods issue qty accordlying.
 

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