Planning Strategy Selection

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.)