For mrp3 in material master, if you set different 'availability check' that system will calculate the ATP in MD04 in different way, so who can help to provide clear logic how SAP calculate the value?

 
The availability check is calculated according to a scope of check defined in customizing.
In a scope of check you define what has to be taken into account during the availability check, depending on the application.

For instance, if you're delivering a material (checking rule A), you shouldn't take into account future goods receipt.

On the other hand, if you're confirming a sales order, you probably will have to take into account future goods receipt and maybe also planned orders.

The scope of check that must be used is determined using the availability check set at material master (MARC-MTVFP) level and the checking rule, which identifies the application (delivery, sales order etc.).

Have a look to transaction OPJJ.

Transaction CO09 calculates, for a given material / checking rule, the ATP. Try it and see the different results. It's also possible to display the scope of check that has been used.
 

Thanks for your answer, I understood a little, but how can I find detail calculating way about different rule?
 

I would like to add some more points :-
The system uses different checking rules based on kind of processing it is doing i.e while sales order creation it uses SD related checking rules and while production related processing it uses PP related checking rules (defined in OPPJ transaction).
Availability check done during sales order creation for material that is sold depends on the planning strategy group maintained for that material. eg :
1) For material with strategy 40, requriement type would be KSV (Sales order with consumption) in the sales order. Hence availability check during sales orders creation will be carried out as per KSV requirement type (i.e. in Sales order, Display Scope of check it will have A - SD order which is mapped to requirement class 050 - Warehouse consumption).

2) When you are converting planned order to production order material availability check is carried out as as per PP Checking rule.
Availability check for material is carried out based on the material master data in the following priority. First it will check for Strategey group in MRP3 view. If no data is maintained in Strategy group then it checks for MRP group and then for MRP Type-Item Cat combination and finally MRP type alone.
Detailed help regarding this is available in
SPRO->Sales and Distribution->Basic Functions->Availability Check and Transfer of Requirements..
 

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