Subcontracting stock reduction

Hi,

In our scenario subcontracting stock is added along with the usual stock and it will be available in total available quanity and SubReq will get reduced from the toal available stock. But we want the Subconstock to be only be decreased when a SubReq comes for the same vendor and it shouldnt take from the total available stock.

Could you please let me know how standard APO planning works (SNP Heuristic).

How to achieve the below requirement:

1) Subconstock shouldnot be included with the usual stock. (It should be seperate from other stock)

2) SubReq should only take from Subcon stock and it shouldnt touch other available stock.

Thanks,

Prash