Selection priority for planning strategy is:

First material, second MRP group and last plant parameters. E.g. System first checks for material, if strategy is defined there, that is used, if it's not maintained there, then it checks if it's defined in MRP group.

That is the same for all MRP parameters, not only strategy.

If you wanna test how it works, try this:

1. create an mrp group, say 0001, and assign strategy group "40 - planning with final assembly" in the MRP group parameters.

2. create a material, say product1 and define planning strategy as "20 - make to order". And also enter the MRP group 0001 in material master.

3. Create a sales order and create an order item for product1.

4. In the sales order header, go to the Procurement tab. There, you will see the requirement type for product1, which is automatically selected by system according to the planning strategy it uses. That requirement type should be "KE" if strategy 20 is selected, or it should be "KSL" if strategy 40 is selected by system.

5. In my test, system selected KE requirement type, which indicates that system used strategy 20, and which I have entered in the material master. It didn't select KSL, which indicates that system uses strategy 40, which is entered in the mrp group. Finally, this test shows that system takes the planning strategy in material master (if it's entered), not the strategy in MRP group.

I think you can now make another test how system uses MRP group's planning strategy. (hint: Delete planing strategy from the material master and repeat the previous test steps. That would end up using strategy 40, which is selected using the MRP group parameters.)

With Compliment: Osman Karslioglu

When do we use planning without final assembly strategy?

We have mainly MTS, MTO, planning with final assembly and planning without final assembly. Now when we are dealing with planning with final assembly, we would usually produce subassemblies (HALBs I guess) using make to stock, so do we use a planning strategy 70 for subassemblies and do a MRP run. To my knowledge I thought we run MRP only for final products (with final assembly) so in that case how would we plan for subassembly products.

I get to understand you are using strategy planning with final assembly. So you are using strategy 40 for the finished product.
 
Just to clarify certain things, MRP can be run for any material...be it finished or semi finished or raw material. Actually when you run a MRP for the finished product using MD02, you are doing a multilevel run.
 
Secondly, when you are using 40 for the finished product you may or may not use any strategy for the semi finished goods. It depends on your business. If you are going to produce semi finished goods independent of the finished goods, you may have a make to stock strategy 10 (one of the many options) and produce them independent of the finished good requirement. Now when a requirement will be generated for the finished good and a MRP run will be taken, it will show that the Semi Finished is either in stock or a planned/production order exists and thus only the remaining quantity will be planned for (there is a better strategy existing for this type of a scenario.....
 
However if you do not put any strategy for the Semi Finished, then planned orders will be created for the Semi Finished for the same quantity as the orders created for the Finished goods.
 
With Compliment: Arvind Khinvesra

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