SAP Planning & Production Alternate Plant

What is the difference between planning plant & production plant? Why do we maintain planning plant?

The planning plant and production plant are used in case of the cross plant planning. Through demand management we can plan across the plants.For each material there is a planning plant and production plant or ordering plant. The planning plant procures and stores the materials independently of orders. There are also one or more ordering plants in which the materials are used for sales orders, production orders, or for other requirements.It can be controlled through the special procurement key in MRP 2 view of the material master.
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Planning plant is the plant in which you don't produce that particular plant (A) but you need them so you plan them and get produced in another plant(B). And this plant also belongs to the same orgainisation.

Now you create a special procuremnt for the Plant A and you maintain in the mateirl master.so whenever demand comes in the Plant (A) it will be planned here and the same requreiemnt will be transfered to plant b and there it will prodcued and transfered to the plant-a.
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Suppose there are two plants A & B in an organisational structure.
Suppose you need a material in plant A but you cannot produce it in plant A because of unavailability of resources. And these resources are available in plant B.

So you can produce the material in plant B and the requirement is thru plant A (e.g thru MRP). Here plant A becomes planning plant & plant B becomes production plant.

To carry out the actual planning and production, follow the following steps-
1) In SPRO, in the special proc key parameter's setting for plant A, mention plant B as the alternative plant.

2) Prepare the material master of the required material in plant A, in MRP 2 view maintain spl proc key as 40 (proc from alt plant) or 80 (production in alternative plant)

3) Maintain the material master in the alternative plant too. In MRP 2 maintain proc type = E. Maintain routing.

4) Carry out MRP run in plant A. If there is any demand, a STPR (spl proc key 40) or planned order (spl proc key 80) will be created in plant A. The stock requirement list will show plant B as the production plant.

5) In case of 40, a planned order will be created in plant B. In case of 80, with proper settings, the planned order in plant A after converting becomes a production order in B.

6) Carry out the production. In case of 40 the material can be sent to plant A from B thru STPO. In 80 the stock in plant A is directly updated when you hit a GR against production order in plant B.