When all tasks in an item are done, the item is considered completed. From there, Planbox gives you optional item states for QA or customer acceptance. Click on the Item Status icon and you get a drop-down menu with those choices.
The Mark as verified state can be used by the Quality Assurance engineer to mark the developed feature or fixed bug as working. The Accept and Reject states can be used by the stakeholder (i.e. end customer) to accept or reject the delivered feature. This is often done during an iteration review. The Release state can indicate that the feature is now in production.
Of course these extra item states are fully optional. But you can take advantage of them to suit your process. And note that they are available for items always - even when an item is in a past iteration.