The stages are a manual process. With the creation of a PO, it must either be saved, or saved and authorized. If it was saved, then you must select the order, then authorize it.
After a PO is authorized, it will move to the awaiting confirmation stage. Once your supplier comes back and says everything is okay on the purchase order, you can confirm the PO.
If the PO is not confirmed and needs to be changed, you can edit the PO and it will move back to awaiting re-authorization, where it will need to be authorized again before moving back into awaiting confirmation. You will then have to confirm it again. After confirming the order, it will move into pending delivery. From here, you can receive or void the order.
Once all items in a PO are delivered, the PO is fulfilled. When only some of the items in the PO are delivered, the PO is partially delivered. Only after the rest of the items are delivered is the PO fulfilled. If you choose not to wait for the rest of the items, you can close short the PO.