In Purchase Requisition Browse (5.1.5), you can see that the system has converted the planned purchase orders into purchase requisitions. Notice the Release Date and Need Date.

The Release Date is when the purchase order should to be created and released to the vendor to ensure that the material will arrive when it is needed (Need Date). The Release Date takes into account the purchase lead time and any internal purchase inspection lead time. The purchase lead time is determined in consultation with the vendor and makes allowance for transport time.