Hi Kumar
I assume that by toll manufacturing you are refering to an operation of a production order which is externally processed, right?
In this case, you have to set the control key 2 on the order operation and system will create a purchase requisition when an order is created. The components may be sent to the vendor, if they are assigned to this operation.
The basic difference is that this requisition is linked to the production order and that there is no goods receipt on the purchase order, as it refers only to the service.
BR
Caetano