The typical Routing can be optionally assigned for any Product Structure if You need to manage manufacturing process for this Product as a sequence of technological Operations. During Planning of new Production Order the duration of manufacturing process for this Production Order will be calculated using specified Ordered Quantity and predefined Operation's details.
If You don't need to manage manufacturing process for this Product as a sequence of technological Operations, then Routing field should be kept as Empty. The duration of new Production Order will be calculated in such case in accordance with Lead Time parameter specified for the Product Structure.