How to set the configuration in order to generate the purchase requisition

HI,

As I know the third party sales the process is Inquiry->Quotation->Sales Order->PR->PO->MIGO->MIRO->Delivery->Invoice.

In this process the purchase requisition is generated automatically when saving the So.

Can any one tell me where we need to do the setting for this automatic PR generation in SO.

Thanks&Regards

Venkat