'Padlocks' icons appear throughout the system and indicate that the row represented is locked and can not be deleted or removed from the current configuration. Locks appear once postings have been made against the object in question. In the case of the project items assigned to the project in the image below, assigned project items can not be unassigned from the project because postings have already been created for the project/item combination.
Note: To effectively remove an item from a project without deleting its assignment, an items status can be set to 'Inactive'. Doing so makes the item unavailable for new postings within the system.