Hello Nobu
Did you make any kind of research about this subject?
There is plenty of information on SAP Help and on old threads.
Basically, the stock provided to the vendor and the sales order stock are both kinds of special stock, therefore, it's not possible to have both at the same time. Therefore, subcontracting requirements are always at plant level.
Take a look, for example, on the old thread Re: 541 E not accepting, where this scenario was discussed. The thread MD04 - Subreq - Storage location MRP area also explains tthat it's also not possible to handle the subcontracting requirements at MRP area level.
Also check the following link from the SAP help, which provides more information from the MM side.
SAP Library - Managing Special Stocks (MM-IM)
I'd like to suggest you to make a more detailed research before opening a thread, since all the links from my answer could be found on google.
See also the following link:
Asking Good Questions in the SCN Discussion Spaces will help you get Good Answers
BR
Caetano
