In addition to actions, we can also capture business rules by means of validation. For example, a Claim, once submitted, cannot be submitted again. In Wicket Objects, this is shown by a tooltip:

Naked Objects supports three different types of validation: whether the class member (property, collection or action) is visible; whether it is usable, and, whether the proposed value or arguments are valid. Or, more pithily, "can you see it, can you use it, can you do it". Wicket Objects surfaces all of these different validation rules in the GUI.