It is based on the receipt date and calculated with respect to the subcontract lead time of the released product.
It can be adjusted when the planned receipt date of the purchase order linked to the subcontract order is modified: a message is displayed suggesting that you adjust the subcontract order start date. If you answer 'Yes', the start date is updated based on the order date. In the case of several purchase orders associated with the subcontract order, this message is not displayed: The start date cannot be updated based on the orders.
This field is managed depending on the setup of the entry transaction of the subcontract order.