The Inventory module in Tekla PowerFab consists of two separate parts: inventory and inventory history. In Inventory, you can take items from stock, return them to stock, reserve items for a customer order, and so on. In Inventory History, you can view the entire history of items in the inventory.
Any item in Tekla PowerFab becomes an inventory item when it is loaded into a purchase order. Note that you cannot modify the properties of these types of items as long as the items have not yet been received.